They are buying why would you think they are not? On low volume days sometimes you can see their bids (30 min. before close they disappear b/c SEC regs require no buying back in last 30 minutes, which is why sometimes the share price falls near the close of trading on low volume).
Your logic is simply not there my friend. The company can't create volume with their buyback, they can only place orders at the bid, they are restricted by SEC rules from simply hitting the ask (for obvious reasons), so they can only buy if someone sells to them. You can see their bids placed as I mentioned, they are probably buying about 20-30k shares a day which is their average over Q2 and Q3, just read the quarterly financial reports and you will see, plus Tom's statements on the conference call. They have to comply with various volume and bid restrictions, plus the low volume makes it hard for them to buy the maximum they are allowed to. Nevertheless, they have bought back a good chunk of shares and continue to do so.
They were buying, but they had to cancel the buy order 30 minutes before the close. When their buy was there, they often cannot have it filled because of low volume. Then after they canceled their order, some low volume sells drop the PPS.
Sometimes I thought about buying more before the close, and hold up the PPS. But, I could get only a few hundred shares, but have to pay commission fees. I would like to grab 3000-share blocks with a single buy.