Hey - i have no further insight as to why this happens - but I have also had buffs not hitting my sk. I thought it was a EQ thing - not a MQ Thing.
Usually it sticks on the second attempt so it wasnt blocked, and i was not at max buffs.
I actually watch for this now - I am casting Bard Dbl Invis and watch for the message to say it cannot take hold.
I will try to run some tests to see if i can duplicate it.