Well Anna,
Ric hit it on the head when he mentioned making a person (instructor included) feeling obliged to dance with one. That is one of the things to which I alluded as among "many other reasons" in my earlier post. Don't put pressure on a person to dance with you. You may come across as too needy and pushy; same goes for guys, so no sexism intended here. (However, sexism may be intended elsewhere.

)
Aside from personal hygeine, which is often a big reason for me to decline dances (politely), I am reluctant to dance with dancers that claim to be able to "follow anything you lead", have no frame, hang on their partners (even Arnold Schwazzenager would get tired carrying a woman around for 3 minutes) or cling too tight, or have no sense of music and rhythm. Instructors sometimes have to dance with such people to further their business, so not every one of the above will apply in that specific case.
Ask him, ie your instructor, again the next time, casually, and without mentioning how many he owes you. If he declines, try once more. And if he declines again, then you have your less-than-subtle hint. MOVE ON. Because, as the colloquialism goes, 'he ain't worth all that'.. 
Good luck, I hope you will find that he enjoys dancing with you as much as you do with him.
About spotting. This is what my instructor told me. Imagine everyone in the room in their undergarments.. Oh wait, that was my debate coach. Well, never mind, it still works for me.. (see what I said about sexism earlier
).
Enjoy dancing.
Sri.