I think Nicole is right. Most likely a water chemistry issue. Sand lasts a long time, I'm on my 10th season. Unless you goop it up somehow there is no need to change. There is a possibility the cloudiness is stuff in suspension that is too small to be caught by the filter but I doubt that. If that is the case there are tricks to add DE to sand filters to handle those issues. Check the water forums for more info and post your numbers. Free or Total Chlorine (FC-TC) Combined Chlorine (CC) if you can measure that, pH, Total Alkalinity (TA), Calcium Hardness (CH) and Stabilizer aka Cyanuric Acid (CYA). OR...post what you can measure and if you can't test all that stuff then take a sample to a pool store. Just stay away from the cash register and turn off your receiver (ears) to what they may tell or sell you.
Al