Two of my PSP UMDs do not work - what do I do?

Two of my PSP UMDs do not work, they are "Everybody�s Golf" and "Wipeout Pure". The problem is that the UMD, when inserted into the console, does not register and there is no option under the drop-down "game " menu to load the game. I presume it is a cleanliness problem considering all my other 9 UMDs work properly. If it is a cleanliness problem, what is the best way to clean UMDs (the casing makes it difficult)? If it is not a cleanliness problem your advice is appreciated. Thank You