It’s okay to not remember things, just enter your best guess and if it is wrong, re-read the mnemonic. You will get it eventually.
Otherwise, try to workout why you are having trouble with readings. Clearly mnemonics work for you as you are remembering the meanings.
Typically the reading mnemonic is linked directly to an English word, which contains the phonetic reading (some times with a bit of a leap).
Are you able to remember the English word tied to the reading? If yes, then maybe you need to study Hiragana/Katakana some more to help you connect the dots. If not, then think about why this could be. Why are you able to use the mnemonic to remember the English phrase for a meaning and not the reading. What is different?
How many items are you having difficulty with, which percentage? Many WK users limit the number of lessons they do per day or per week, maybe that is worth a try?
Lastly, I strongly suggest reading into mnemonics and how they work. The mental image needs to be vivid. The more ridiculous, the better. If you are having difficulty with the WK mnemonics, try making your own.