1995 gmc diesel pickup, randomly wont start

my 1995 gmc diesel truck runs great but every few weeks I will drive it someplace, park and when i try and start it, it just turns but won’t catch. I come back the next day. it will start. the computer shows nothing. it always starts at the garage for the mechanics. they have checked wiring, replaced both batteries, added fuel cleaners to diesel. any ideas?

You don’t mention any checking of the glow pugs. The TSB’s are running pretty good towards a glow plug issue.