There are locks and adjustments for everything, but I have seen certain of these features worn out on very heavily used old enlargers. A bit
of the problem answering is that the 138 series was made over several decades, and not all of them are exactly the same. With my units there are adjustment screws on each side; but due to the age of one of my 138's (which I originally bought for spare parts and later refurbished into another fully workable unit), I had to turn the front locking knob rather aggressively tight to avoid the problem you describe. ... it annoyed me
and I eventually found unused parts for a true cure. You really gotta inspect everything and look for signs of excessive wear and tear.