Typical problem on Stern drop mechs. The mech seems to work fine when you manually push up the bar. But the solenoid pulls the plunger, not pushes it. The linkages got worn down over the years so there is some slop. Also, the reset bar may be bent down some. The result is the solenoid cannot pull the targets up enough to lock. I have not found a replacement link for these yet.
I superglued a zip tie on top of the reset bar to make up for the slack. It adds that 1\16" to 1/8" needed to push the targets up higher to lock. There are probably other options but this was quick, cheap and works so far.