How do you find out what is causing your credit to be bad?

My husband didn't have any credit (not good, not bad). He took a loan out for a truck and we have paid it on time in full every month. When I had my daughter the hospital (like most) screwed up our insurance. I had two insurances at the time and instead of billing ONLY the one that covered my entire pregnancy, they billed both. They started sending us bills. I explained to multiple people their mistake but they sent it to specialized management services anyways. They ended up sending it to the credit companies and therefore my husband ended up with a bad mark. Finally I got hold of the right person at SMS and they took it off. Ever since my husband turned 18 he hasn't been able to get a credit card because he never had "enough credit history". Recently we were sent something to apply for a card and they denied us because of "delinquent past and present obligations"? I'm not quite sure what that means. We want my husband to have good credit. All of our bills have always been paid on time and in full and we don't understand why his credit would keep coming back not good enough? Any suggestions?