So a few months back I bought a used TS-44 Pro buttset off eBay. It appeared to be in mint condition. The line cords were in perfect shape. Bed-of-nails perfect, etc. It worked fine but now one of the batteries (I think the primary 9V) needs to be replaced. No audio from speaker, etc. It has 4 screws on the backside of it to access the battery. I unscrewed them per the instruction manual (screws don't come all the way out per manual), but the door will not come off. I tried pulling it in all directions. I even knocked it on the edge of the concrete floor to see if that would loosen it, but no dice. Does not budge one smidgen. Would rather not break out the dremel computerprobs

What am I doing wrong?