is the problem that the fret slots are too tight for your fret tangs ? - i have found that on hard woods that it takes a lot more effort to install frets - i usually file the barbs on the fret tang when i'm using a hardwood fretboard such as cocobolo and find they go home a lot better - i have seen a fret barb removing tool (think it was at stew mac but dont quote me on that ) which removes some of the barb when you pull/draw the fretwire through it
another idea may be to slightly taper the bottom of the fret tang rather than adding the taper to the fret slots
also is it the same fretwire you have used before because some have a variation in tang thickness ?
Thats a strange one - i can't see why adding a slight bevel to the fret slot would cause such problems - i always make my fretboards and fret them before i glue them to the neck
fretting oak for me has never been a problem the frets usually just pop straight in
infact the only problem i ever had with fretting was the saw i was using (brass Back From Stew Mac) lost its teeth offset pretty quick which did infact make the fret slots tighter maybe this is the case - you could try just cutting a fresh slot in a off-cut of the same wood you have used for your fretboard and try fitting some fretwire without adding the beveled edge to the fret slot and see how that goes !
BTW i now use the japaneese Pull saw from stew mac it's much better than the standard Brass backed type
thats about all i can offer mate hope you get it sorted
cool mate at least you know what the problem was :) - if your going to buy your new saw from stew mac i would suggest buying the japaneese pull saw it also accepts the depth stop and its only a few quid more than the standard Brass Backed saw they sell - i have both and the japaneese is far better IMO
I mildly (very slightly) radius the slot, especially in harder woods to compensate for the slight filet or radius in the underside of the fret wire, it seems to help make sure the frets lay flatter (and stay there.) But I agree that it makes the wire a little trickier to get started straight. One thing that helps me is to leave a little more of a wire "tail" to hold as I start them into the slot. And yes, as noted-the right saw makes a big difference.
Bevellling the slots, very lightly, can seem a useless exercise unless you want to pull the frets out again - causing splits and tiny shards of wood to come away with the fret, ruining an otherwise perfect slot! )-:
I usually lightly bevel along the slot using a triangular needle file, place the end of the fret wire strip along in the slot then push in with my thumb, then when sure it is "in" gently tapping with a small nylon hammer - then snip off the ends and a few more taps with the fret hammer... done - only 21 or so to go! (-:
You may be able to have your old saw sharpened. Sounds like it is a cool one with a history.