Often this situation will be created by Merge inventing spouseless relationships.

Try this: Get to your maternal grandparent's Family Page, click on your mother, then People/Other Parents. If you see multiple sets of parents for your mother, that is no doubt your problem. Do the same from your father's perspective. Incidentally, the parenthetical abbreviations after the names indicate the sort of relationship, N=Natural, U=Unknown, &c 

To clean this up, you will want to do things like: Merge individuals (if there are multiple definitions of one or more grandparents, for example), detach child (if your parent is attached in the wrong places), &c.