I had the same problem with my scope, i saw ferns and spots one day then nothing for two days (just spots) and then ferns and spots again and now nothing since
I lick mine and then put it on my dressing table to dry for about 15 minutes and then i read it, my instructions were very vague so i asked my chemist, he said you can lick it or use you inner cheek bit or your cm, personally i use my saliva, but either works, he said it best not to use your finger as it may interfere with the reading!
Im not sure if you should put a certain amount of saliva on it though! Sometimes mine only shows up 2 spots when i lick it and other times it shows up loads, so im not 100% how much is 'enough'.
Hope this has helped a little
p.s the spotting and ferns is called the Transition phase.