I routinely use FD flat-end blades on my Hawk 220 and don't have any problem holding them. I feed from the bottom and once the blade is in the bottom holder sometimes it takes just a very slight finger-twist of the blade to make sure it lines up perfectly for the upper holder. I do find that it takes a very positive twist on the tightening screw (I wish it were a round knob) to make sure it doesn't slip out. I'm currently using some Plain End Sharktooth BLDFF20 blades that are working very well with no problems.
Whenever I do have a problem with blades slipping out of the upper holder, I find that a couple of strokes on the end of the holding screw on very find sandpaper solves the problem.
Dennis