I saw a thread on the same subject some months ago, either here or on the LF Q&A. The poster found that his sheet Tri-X was underdeveloping by about a Zone (N-1 instead of Normal with his usual Normal development). At the time I ignored it and so did most everyone else.
Recently I had the same problem with Tri-X in 4x5; i.e., with my Normal times it seemed the negs were all just slightly more than N-1 developed. Grade 3 paper did the trick for me, but I went searching for that thread... The poster had done a bit of densitometry and I wanted to see what his results were and maybe contact him. Never could find the post.
I have increased my developing times for Tri-Xd 10-15% for all development schemes and things seem to be alright now, but I haven't had much of a chance to print the new negs. I should really re-calibrate (a job for this summer when I get back to the darkroom in the States).
The OP for this thread may have had a similar experience.