The reason why it's harder is because the more stats u add together the higher the glyph counts
So if you're rank is rank 5 (Magical T2 should be ur shown minimum)
Now at this point u have 90% successrate (should have) and that is for Magical and Rares (Mystical-mythical = 60-70% if i remember correct)
So when you merge 2 items together, let's say 2x rares for example, rare t2 glyph with life + mana and you want to merge that together with another rare t2 glyph that has int + xp
Now this glyph will count as a Angelic because the total amount of stats equals to 4. (now that is above ur rank, which makes it harder = more ?!?!?)
These ?!?!?! can also remove the full bar sometimes, and they still pop up for me who have rank 45 on tier 3 items.
Yes I do fail t3-t4s even at my rank which SUCKS