Private reg

I bought a car from a dealership recently. Prior to buying the car, I had a private plate which I wanted on the car so the dealer told me to send him a cheque made out to DVLA and complete the transfer plate form and he would send everything off alongside the new keeper details, tax and mot certificate so when the new V5 arrived it would show the new plate.

Bought the car on 28th June this year and yesterday I received the V5 but it's still showing the old plate and not my private plate..anyone know whats happened?! I sent the dealership the original retention certificate and don't have a copy to sort it out myself. When I went to pick up the car, he reassured me over again that he had sent everything off to DVLA, photocopy of tax disc, mot, new keeper details, retention certificate and cheque I had made payable to DVLA for £80 to move the private plate on to the vehicle.

I am kinda confused as to why the V5 has arrived showing the old plate that the car was sold with, rather than my private plate.

Does this mean I have to send it off again, if so..how do I go about getting another retention certificate to evidence that I own the plate ( I had to previously request one so unsure if they will just send me out another one).

I'm kinda pxxxd off with dealership who assured me they would sort it out.

Any advice, suggestions would be highly appreciated. Thanking you all in advance xx