I doubt it was a scam. It was far more likely to have simply been an inefficient company, jumping on the PPI bandwagon, that either did nothing with your form or (possibly more likely) couldn't find a way of getting your PPI payments (if any) refunded and just couldn't be bothered to let you know.
It's probably akin to the vast number of companies who jumped on the 'free cavity wall insulation' bandwagon. I signed up with one of them but then never heard any more. Eventually, only shortly before the Government scheme ended, a doorstep caller invited me to sign up with their company instead and, as I'd not hard from the first one for over a year, I agreed to do so. They sent someone to drill some test holes in my wall and agreed that I qualified for the free insulation. However when I later chased them up about why nobody had installed it, they said that the deadline had passed and I couldn't have my free insulation after all :(
The PPI claim deadline has now also passed, so there would seem to be little point in worrying too much about it. Despite the vast number of companies involved in the business, and the millions of people who made claims through them, I never heard of any of the firms getting the repayments sent to them and then simply sitting on them, so I think it's unlikely that you've been scammed.
Incidentally, I nearly ignored a letter I got (from a company I'd never heard of) inviting me to apply for a PPI refund; I just assumed it was yet another firm trying o get me to sign up to a service I didn't think I needed. It was only many months after I'd received it, when I was just about to finally chuck it into the bin, that I noticed that it referred specifically to my old Monument credit card, which a random mailshot would not have done. I looked into it a bit further and found out that the unknown company was actually the successor to Monument Bank. So, purely out of curiosity rather than anything else (as I'd no real intention of making a claim, as I didn't think I was owed anything) I went to the website mentioned in the letter and typed in the reference number I'd been given. I was expecting to be asked loads of questions but all I saw on the screen was "Thank you. Your claim is now being processed". A few weeks later, without any further action from me, I received a cheque for just over £3k ;-)