I am not surprised that the manual stapler cannot drive the composite staples staples into Douglas Fir. I have not used composite staples but have driven many hundreds of thousands of metal staples when I was a professional boatbuilder at the beginning of my carreer. I found hand staplers were only adequate to lightly hold components in position. They were not capable of driving 3/8" (9.5mm) firmly into Douglas Fir. I built a handful of boats with an electric stapler that was capable of driving 3/8" staples but not 9/16" (14mm) staples into Douglas Fir. Pneumatic staplers are capable of driving 9/16" metal staples even into White Oak.
I can no longer source the 9/16" Silicon Bronze staples I used to use and now use Stainless Steel staples for my extracurricular boat building. They are relatively inexpensive compared to the bronze staples I used to use or the new composite staples. I almost always remove them after the glue can hold the components together. But if I break one I just use a nail set to drive the leg below the surface and leave it in place. There are three tricks for removal. 1st, dirive the staple through a sacraficial strip of wood (I usually use popsicle sticks, 150 sticks per USD). 2nd, don't wait until the glue is completely cured. 3rd, use a good staple remover. An added advantage of using the sacraficial strip is that the staple doesn't scar the wood too much.
I attached a photo of the system I use today showing: A hand stapler; A pneumatic Stapler; A box of Stainless Steel staples; A staple remover; A package and a half of popsicle sticks.
