why do some of my app icons disappear from time to time?

[Michael Mcgraw]

Not sure if this is an appropriate venue for this question, as I'm new...but I'm curious if anyone can tell me why some of my app icons disappear from time to time? Seems to happen when I restart me phone sometimes. They're still available for me to replace, but some I have replaced some as many as 4 or more times in a month. Phone is an HTC One V...thanks