Hi Shug1067
Welcome to the forum.
You might want to have a look at the following FAQ which explains where everything is;
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum .
Did you send the form in by recorded delivery, if you did check when it was delivered and who signed for it, I would take a screen shot as proof, then contact DWP and relay the information, sometimes post can be delayed from receipt to being added on to the DWP system.
If you did not send it by recorded delivery, did you send it by proof of posting? if not you can only keep trying, unfortunately some post does go missing as DWP received 100's of pieces of mail each day.
Gary