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You may want to check if you can actually see the proximity sensor to the right of the phone earpiece - if you're using a screen protector, make sure that it is clean as it may be obstructing the proximity sensor. If you don't have a screen protector, just make sure that the proximity sensor is clean.
If you can't see the proximity sensor, then you may have an older variant of the Lumia 535 with the known touch screen issues - have your screen replaced in a service center while your phone is still under warranty.