@Joe ******
you can only enter without a visa at a land border 2 times per year. There's no limit written into law for arriving by air. But after 2-3 times or 4-5 months they're probably going to start questioning you and possibly give you a warning before denying entry.
If you applied for the multiple entry tourist visa that's probably what's wrong. You have to prove you need multiple entries. That means uploading a ticket to Thailand, a ticket out of Thailand within 60 days, and then another ticket into Thailand. On the visa application they say you must show your justification for needing multiple entries.