On Amazon you can find things to make the buckle stay upright to help them secure it on their own.
There’s a group on Facebook called Car Seats for Littles that can help troubleshoot issues like this. You can post photos any stuff
My 7 year old recently switched to a bottom booster form a high back & this has been an issue.. we got buckle extenders & these things that help keep them in place & it’s helped a lot.