+1. Were you literally holding the neck in the air with one hand while trying to knock a fret in with a hammer in the other hand?

The neck needs to be solidly supported on something that can absorb the energy of the hammer hit and keep the neck from being knocked away from the hammer or recoiling back toward the hammer. If it's not attached to the body yet, it would help to clamp it to your bench. That would allow you to use both hands, one for the hammer and the other to hold the fret in the optimal position for it to go in cleanly. Something like the sandbag in the photo below works great. It's just a jeans pant leg filled with sand. The sand is inside a trash bag inside the pant leg. Easy to make and cheap. It conforms to the shape of the back of the neck to provide good support and the sand absorbs the impact.

Did you chamfer the edges of the slots before trying to put the frets in?

All the fretboards I've done so far have been ebony and the frets went in easily.

First good on you Cablepuller for fretting the board on the guitar! That's how we do it and how refrets have to be done so good experience for you AND you have more control over shaping the fret board prior to installing frets too.

When I first started out we used to joke a lot on this forum because my first shop was in a bathroom in my condo... My second shop was in a spare bedroom with plush, white carpet..... I didn't care.... to hell with anything that was not a guitar!

Anyway long story short I had trouble hammering my frets too. Turns out that my bench legs being on thick carpet were absorbing the blows of the hammer.... It was not until I used something immobile to tap on that I had any success.

Next you mentioned a plastic hammer. For tapping in frets, notice I did not say "hammering" in frets... a dead blow hammer is what you want. Other hammers bounce too much to be suitable and this may be one of the issues that you are experiencing.

The guys are right too on supporting the neck well and a 25 pound bag of bird shot is what I used to use.

Food for thought. We press frets and tap with a hammer too depending on what method gets us where we want to go. We greatly prefer pressing frets though and this is what I wanted to suggest to you too. Fret press tools can be made, purchased, etc. but we believe that pressing is easier, more consistent and with a little bit of thought you can use a proper fret press tool such as the Stew-Mac Jaws II tool to fret all frets on an acoustic regardless of location.

Regarding the beveling of the edges of the fret slots. There are two reasons why some do this. First most fret wire has a filet at the transition from the tang to the underside of the fret. This filet won't seat well unless you bevel the fret slot edges.

We don't bevel the fret slot edges because we use Jescar wire and if you look at it under magnification the transition with Jescar wire is much cleaner without that filet not permitting the wire to seat completely without the bevels.

Second it's believed that come refret time that the bevel will assist in less chipping of the board when gently.... "lifting" the frets out of the slot. I agree with this but the bevel is not the only way to prevent chipping come refret time. We use very, very thin and fresh CA to glue and clamp each fret. When you apply a bead next to the fret if the CA is thin enough you can see it wick under the fret tang and wet the opposite side of the fret board. Clamp that puppy, spray accelerator and the fret is firmly glued in place AND the fret slot itself has been for lack of a better word stratified and reinforced also preventing chipping come removal time.

Hope something here helps you out!

Cablepuller - I get the impression from your original post that the neck might not yet be attached to the body. Is that correct?

The reason I asked is how to support the fretboard extension with the neck off the body would have to be thought out well. A sandbag like the one in the photo above wouldn't work because it would not fit all the way into the corner between the underside of the extension and the back of the heel. Maybe someone here installs the frets with the fretboard glued to the neck but with the neck off the guitar and could give you an idea how to support the extension.

People have different approaches for when they install the frets. I was taught to install the frets with the neck on the body. In my case, permanently attached, but I don't see why it couldn't be done with it temporarily attached. The important thing when doing it this way, is to provide support under the extension inside the body to absorb the energy of the hammer blows so they don't damage the top in that area. There are some elaborate ways and some simple ways to do that. I hold a compact, heavy piece of metal (a metal working dolly) firmly against the underside of the soundboard between the head block and the upper face brace. The energy beyond what is needed to seat the fret is transferred through the top and is absorbed by the dolly so the top is undamaged.

Edit: Thinking about more, I'm not sure having the neck temporarily attached would be safe since the extension might have a small gap under it meaning it wouldn't be solidly supported. Even if there was no gap, it wouldn't be securely attached to the surface of the soundboard and there might be some risk because of that.