I had the same problem and in my case its the ants fault. They went crazy for the sand and most of their hills either blew or got kicked into the pool. I had the patio guy come back and redo half of it the second year thinking it settled. Then 2 years later i pulled and redid the other half. This summer there was no sand left and it dawned on me there were no ants either. The whole patio is down 1/2 - 3/4 of an inch and now that its uniform it looks so much better than when it was wavy all over. The only tiles that didnt sink were the 4 corners. The bonding rebar is holding them up where they started. This summer i plan on chipping out the backs of them to make them match the rest of the patio. The part of the patio that i fixed was backbreaking and being my first time doing so, didnt come out a nice as i would have liked. Then it only lasted a season and it was right back to where it started. I used a hand tamper and the ants had a much easier time ripping it back up.
I tried several ant sprays and traps but never found the right combo. Or it helped but they just moved 10 feet over. If i end up pulling the patio again i will use some sort of cement instead of sand. Preferably i'd watch whoever i hired do it but i have many other projects on the list right now so it would most likely be me. And it was so miserable to do that im not in any rush to try again.