There's a lot that can get off kilter in a family dynamic, whether you're talking about two generations or three.
Whether grandparents are regular fixtures in their grandchildren's lives or only occasional visitors, a whole host of interpersonal issues can come up. A grandparent might fasten onto a favorite and ignore other grandchildren, for example. Or criticize parents in front of their kids. Or perhaps grandparents start to feel taken advantage of when being called upon frequently to care for their grandchildren.
Alternatively, parents or children might feel obligated to arrange time for get-togethers and activities.
The key? Communication - between parents, children and grandparents alike is usually the best way to clear the air or get all parties on the same page. Respecting the personal needs and wants of various family members is fundamental, valuable and the most loving thing you can do.