i think it is natural to blank out once in a while because your mind might not be where it should be but if this is a common occurrence when you are using katakana then it is a problem.
i learnt katakana in 5 days i think it was, i did it in my lunch break at work. i just drilled 10 or so over and over and they stuck in there but for a while there i would draw blanks on ones like ヌ nu because i rarely saw it used.
below is a nice long list of words written in katakana, learn this words and write them over as you learn them, then the characters will come to you faster. i think learning them in vocabulary is the best way to really grind them into your mind
finally sorry about the double mentioning of katakana, it is the way i have it listed in my vocab program, this is a direct export from it
apparently my list was too long and it rejected it, so i cropped from タ down. damn smiley faces.. there should be a code option so no smiley faces come out.