This is the F12 IR protocol, which has an unusual format. The up and down signals you're learned are intended to be continuous signals-- the curtain will keep moving as long as you hold the button. We know this because the 4th bit in the signal is a 1. The form of the continuous signal should be: (12 bits of data, 34 mSec of delay, the same 12 bits of data, 88mSec of delay) repeated for as long as you hold the button. It appears that your learns for the held down button have the correct form, so it is a bit of a mystery why the learns don't work.

Recently there have been two similar cases discussed at hifi-remote.com. Seemingly, the solution was to change the the timing of the IR signal very slightly. I'll read through those again, and see if there are some variations for you to try.