351 Windsor won't hold timing

Hello. We have a 1993 F150 2 wheel drive pick up with a brand new 351 Windsor engine. It's been in the shop for over a year so the mechanic could put the new engine in and get it running.
We just got it back as they could not get the timing to hold. We now have it at another mechanic shop trying to figure out what the issue is.
Any ideas, suggestions or what the problem is?
Thanks in advance.