I bought my pool sand from Leslie's last year (when I converted from Bacquacil to Chlorine, thanks to the encouragement here). I faced the same problem. Some sand on the floor. I suspect some fine sand will be present in any new sand we put in the filter. Simple vacuuming did not solve my problem. It came coming back. I vaccumed-to-waste (filter setting on waste as I vaccumed quickly enough so that I dont lose a lot of water). And I never saw the sand again that season.
This season, I had a problem with high-TA. So I installed a cheap sprinkler at the return to aerate. I saw quite a bit of sand again in the bottom. I suspect the increased water pressure pulled some more fine sand that was otherwise locked in. I vacuumed it to waste - and have not seen it again for last 4-5 days. (pump runs 10 hours every day).
Your problem may be similar. Some fine sand that you can hopefully vacuum-to-waste. If more keeps coming in, then you should suspect something more serious - like cracked laterals.