Typically in chopping the mortises I use in joined furniture, I get the bulk of the mortise down to the depth I want (1 1/2″) and then find that there’s a bunch of un-chopped junk near the ends. Here you can see what I mean in the left-hand end in this picture – I was going for the 1 1/2″ depth, and at the point where the ruler is I am not yet there.
So the thing to do is get in there & cut that stuff up, then lift the waste out on the back of the chisel. I drove the chisel straight in, and then moved it forward a bit, and repeated. this breaks up the stuff at the bottom of the mortise.
After chopping into that stuff, then I insert the chisel bevel down, from the other end. Now I can get under that stuff, and bring it up & out.
This was the 2nd of these “mortises under glass” and it really is helpful to see the action. A little tricky to chop a mortise with a piece of plate glass in the way…but the price you pay for learning.