some machines require the thread to come off the spool from the back, flipping may not work. the Cap must be larger than the end of the spool to help. I think i heard of someone putting clear nail polish on the slit... not sure. i have an old emery board I sometimes use to file things smooth. but lettering, all i can suggest is the cap being larger.
if all else fails, maybe you can set the thread on a thread holder on the machine table itself. like we do with larger spools. good luck.