Why does my iPad screen feel sticky?

Lately, I've noticed that my iPad's touch screen feels sticky to touch which makes it difficult to swipe in a smooth motion (particular when several swipes in quick succession have to be done in quick succession). As I don't use a screen protector, I initially suspected that this could be due to the build up of oils on the screen but wiping the screen with a microfiber cloth (even with a damp one) does little to help the problem. The device is 4 months old at the time of writing; the screen should still be in reasonable shape. What could be the problem here and what can I do to fix it?