Sometimes recommenders really are just too busy to respond, but if you had a good relationship with them then it shouldn't be a problem. The more passionate your recommender is about making sure you get in, the better it is because that passion will show in the writing. If you are unsure about their committment that's not a good sign and may result in less than optimal recs.

Email them and make sure you make the process easy for them. Perhaps you can provide specific talking points and examples from your work that highlight them. That makes the mental work easier for them.

I had a great relationship with all three of them. I just e-mailed them explicit directions, a short list of my accomplishments, a current resume, and offered to assist in any way I could. Thank you.

Be careful - some schools only want 2 and only 2 recs. Make sure you're following instructions. =)