Just because you have slots enabled doesn't mean you have DS30 loops available for the card. There are six loops available (or 5 if using a 3/5 split as Ace mentioned). What other cards are in the sytstem? If this is the fifth DSM, you could very well be out of resources assuming you have four DSM32's and analog/T1 trunks.
If you have four DSM 32+'s and only one T-1, you have room. If you have four DSM 32+, and two T-1's. you're maxed out. Same goes if you have one T-1 and some analog trunks or analog stations.