I have two of these Stanley / Bostick model B440 staplers and they both started working erratically. Even though I had plenty of standard staples, i bought some Bostick standard staples because it says so on the bottom of them. That's not necessary.
The problem was each time I used the stapler once, it would not work the next time I used it. Here is what I found and how I made it work every time.
Step 1: OPEN STAPLER
Open stapler as you would to add staples.
Step 2: THE FIX
You will see a long spring there which serves double duty:
1). It pushes the staples to the front of the stapler.
2). It pulls down on a pivot that pushes up on the handle to allow it to feed the next staple into position to be used.
If this spring is old and weak, like mine, it needs more spring tension. The easiest way to do that is to add a full clip of staples (or as many as it will hold). This stretches out the spring, increasing tension, pulling harder on the little pivot, and pushes the handle away from the part holding the clip of staples. Then the darn thing works as it should, or you can go out and buy a new stapler. Just remember that to buy a $20.00 stapler, you must make about $30.00 so you will have the $20 left after you give the IRS $10.00 taxes.