it depends why the buyer is unable. was it financing, property condition, change of circumstance out of buyer's control (loss of job, death of spouse)??
In some cases the seller can bring suit for specific performance by the buyer. What that means is the seller can sue the buyer to complete the purchase.
why is the seller not signing the cancellation form? I would disagree with Ron and not pursue anything until there is a signed written cancelation from the seller canceling the deal with no further obligation on buyer or seller. Like Ron states, your deposit may be lost.
I would have your attorney on speed dial. If the seller does not sign they may be preparing a suit at which time you need to get your attorney involved.
I am not an attorney and not in any way suggesting or providing any legal advice to you. However, these are possible seller's options depending on why you can't complete the transaction.