like said above make sure you have the right size--
I wouldn't mix 26.0 bars with 25.4 stem for example--
some do but if that **** cracks then you will too.

Nitto makes a tool for spreading the stem temporarily
to let you slide it in. I use a large blunt wood chisel.
Wedge it into the gap where the stem opens for the bolt
with the bolt out and GENTLY pry just a smidge. You only
need a few mm of clearance. Bars go right in.

Another trick on some stems is that if both sides of the
stem around the bolt hole are threaded then unthread
the allen bolt until the end is in the opening between the
two sides of the stem. Then put a penny or another coin
over the hole--thread the bolt back in. Because it can't
thread into the hole and tighten it has the opposite effect--
it forces the stem open slightly.

Don't force anything too much, if it is too hard you may have the wrong
sizes. I had a shop once sell me an old triple T stem that was the
wrong size because it was NOS and they didn't really know what it was.