Hi and welcome to Newlifegames.
This site is primarily for "newer" (non full mechanical) slot machine, BUT there are a few of us experienced in that area.
I moved your post to "Other Reel Games" and I'll inform our resident PRO about your problem.
My first guess is your bottom coin slide is jammed or gummed up. Sometimes people put oil or grease on them, which is a no on.
Can you CAREFULLY pull out the mechanism, and take a couple photos: a good close up of the front of the mechanism, and one of the front bottom right as well? There may be a thumb screw holding it in, so check the lower area. Pull the mech straight out and be careful with the reel strips.
There is another web site that's intended for all mechanics machines "coinopcollectorforum.com" However it's VERY slow with response and you many not hear anything for awhile! If you decide to join that, PM me for the code they will ask.
But let's start here first! We may be able to help you out faster.