Until they establish a good "seasoning" stuff will stick to them. Some things will stick a little even in a well seasoned skillet. Depending on how often you use it a and what you cook in it depends on how long it will take to really have a good seasoning.
What I do in the beginning with a freshly seasoned skillet is wipe it out after use. Once it's used I put it on a burner and spray a little "Pam" on it wipe it in good. Let it get hot then shut it off and let it cool. Once you get a good seasoning this isn't necessary.

Also some high acidic foods can hurt the seasoning. So if your skillet was working out good then all the sudden started sticking something could have gotten to it. Spaghetti sauce and other tomato base products can take the seasoning off if you let it sit in there for a long time.