How to remove plastic screw covers that won't unscrew

<p style="text-align: justify;">I put together an IKEA dresser incorrectly and need to take apart the top board but can&#039;t get the screw covers off. I&#039;ve tried prying with a screwdriver, jiggeling them around, hamnering on the back so they&#039;ll fall out and nothing works... I removed other ones with no prob. Thanks!</p>