Went through this with Bay Area Dave a while back.
He, and perhaps you, overlooked a parameter - bit
slip. Dovetail bits want to "climb" out of the router
collet, making deeper cuts. Doing dovetails, blind
or through on a router table the easy to remember
rule is Heighten To Tighten, Lower To Loosen. If
the bit slips and "climbs" you end up with tight
or too tight joints.
Check the collet for crud, dings etc. as well as the
bit. Clean 'em up, tighten the collet firmly and
you're problem may go away.
You could be setting the bit too low in the collet
and the collet's trying to grab in the tapered
part of the bit below the lower cutting edge. Since
the collet has a very limited range to work
properly, a little movement down the taper can
raise hell with dovetails.
Set the bit a little higher int he collet and then
adjust the depth of cut.
You''re parts may have a slight cup or twist.
Doesn't take much to go from snug to too tight.
The ends may not be square
You might want to look into a lock miter bit
Like many things in life, what seems to be simple
often isn't - until AFTER you figure out what
you've overlooked the first half dozen times.