You don't say where in the process that first offer was. Had the bank come back and agreed to the offer? (I kind of doubt it.)
It doesn't matter whether the seller accepts your offer. The seller will receive nothing from the sale. What matters is the bank--their lender--accepting your offer. That's where the holdup is.
What could be the delay? First, the bank has to determine the value of the house. And it has to confirm that the sellers are experiencing a hardship. That may or may not have been done by the time they dropped out and you stepped in. That's usually the biggest barrier.
Second, the bank has to review your offer. If your offer was the same as the first buyers, or higher, then that's not an additional problem. But if your offer was less than the first buyers, it well could be.
Example: The sellers bought the house for $300,000. The first short sale offer was $200,000. The bank has a BPO (broker's price opinion--sort of like an appraisal) done and the BPO says the house is worth $220,000. In that case, the bank wouldn't have accepted the first offer. And won't accept yours, either, if your offer is under $220,000.
Example: Same scenario as above, but the sellers fail to prove hardship. In that case, the bank really won't consider a short sale at any price.
Banks are notorious for not communicating with the various parties in a short sale. They're also well-known for losing documents provided by both the buyers and sellers . . . often losing the same documents 3, 4, 5, or more times.
So there are lots of reasons why it could be taking long. (By the way, with a short sale, 4-5 months is fairly typical. 8-10 months isn't too unusual. So, with those numbers in mind, maybe there really isn't a delay at all.)
Make sure your Realtor stays in regular contact with the listing agent.
Hope that helps.