They are a simple, basic machine ...
You can't get a more basic machine, except maybe a drill press. The round drill fits into the chuck, a square holed brace secures the chisel.
A large gear rack and pinion move it up or down. An adjustable clamp holds the work down... very few moving parts to break or wear out.
Here's some advice based on my experience with making mortises. I own a 3/4 HP Powermatic bench mortiser, one of the best on the market. When I had a slew of mortises to make for a Mission quilt rack, I used a router with a self centering jig I built myself because it was so much faster and easier. I had to square up the corners, but that went quickly with a sharp chisel. I don't plan on using the Powermatic much ....... The larger the mortise, the more force it requires to chisel the first hole. Succeeding holes adjacent to the first one are easier. Watch some you tubes on the process.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)