pip2 form lost in dwp system

Hi, I'm in the process of being transferred from DLA to PIPs. I sent my completed pip2 form back to the dwp on the 12/4/19 recorded delivery and it was sign for at their address on the 15/4/19. the deadline date was the 29/4/19.
I didn't receive a text to my mobile phone saying they had received it so on the 29/4/19 i rang the dwp. they confirmed nothing had showed up on the system. I gave them the tracking number and they said they would send a request to the post handling department to look for it. I rang again a week later and they confirmed they were still looking for it. I still not received a text confirming they had it in there system yet.
The question is should i do anything else or just wait for the dwp to get back to me. Love to know if anyone else has had a similar experience.
Thanks Tim

Hi Gordon, I unfortunately didn't make a copy of the pip2 itself. The reason because i find it difficult to write i filled it in by ticking the boxes and referring to additional sheets as far as the answers where concerned. I've got copies of these of course and my supporting evidence.
Timnok

Hi Gordon, The day after your post i phoned the dwp. The call handler confirmed they had a note of the recorded delivery ref number . She said she would send a message asking about progress and that someone would ring me back. No one has yet.
I did ask her what happens if they can't find it, she said that they would probably ask me to fill in another pip2 form. She gave the impression that that there was no doubt they had received it.
I'm just wondering if i should send a copy of the proof of postage and screen shot of the proof of delivery?
Should i still be worried, is there anything else i can do.
Thx