I’ve got a 2001 24 valve Cummins with a juice with attitude and a bigger turbo not sure if the size but bigger than stock. It wont go over 17 psi even on tune 5. Not sure what the deal is

 

My first guess you be a big boost leak.
The lower two boots at the intercooler tend to rub the inner fenders and get a big gash in them over the years. It would be the first place I would check if it was my truck.

Also check that the turbo wheel doesn't have too much play in it and is in need of replacement.