At this point you might consider going to small claims.
First, just filing the paperwork will possibly cause them to come up with the money immediately to avoid the aggravation of going to court.
Second, the landlord has to either return the deposit or give you an accounting in 30 days - they missed their date, so they will have no standing in court - the judgement will be for you.
Third, if you can document and prove actual damages because they failed to live up to their obligations, you can attempt to recover that amount in small claims; you can't do that any other way.
Remember, actual damages would be something like "I had to put the amount on my credit card, and here are the interest charges". You won't be able to recover on something like "I couldn't get a job because I couldn't move into my new apartment" because that is iffy, and you can't document the actual losses.
Your chances of succeeding are very good because the burden of proof is on the landlord, not you. Make sure you document everything, and don't conduct any communication with the landlord verbally from here on - it has to be registered mail and everything in writing.
Free legal advice is appropriate in this case because big dollars are not involved, but you should take it on yourself to read up on tenant/landlord law. There are Nolo books at the library that cover this exact situation, and they are easy to read. They will explain in plain English what steps you need to take next. No lawyers are allowed in small claims.
If the judge determines that the landlord withheld funds and did not refund or account for damages within the 30 days, you may be able to ask for the full amount of the deposit back for 'bad-faith' on the part of the landlord, plus actual expenses/damages caused by not returning the money or accounting for it on time. The excuse 'the check's in the mail' can only hold weight if they sent it certified mail, or it got returned with insufficient postage - they'd better be able to come up with physical evidence that it was mailed within the 30 day period or the judgement will go in your favor.
If you study the Nolo book, you'll see where it says you should send a registered letter with a Demand Notice for the full amount (they missed the 30 day limit, right?) or you'll take legal action. It's time to stop fooling around. If they say they want more time to come up with an accounting, forget it - they missed their deadline. If they want to settle with you at that point with a reasonable amount, then take it. Either way, you'll have pushed them off a dead stop.