MBP7,1 keeps showing the "prohibited" (circle with line through it) symbol on boot (yes, I'm selecting the EFI first) but not consistently. It goes away sometimes after a PRAM reset and lets me boot normally

Why would this be happening? I've reapplied OpenCore many times; I've booted from an OCLP USB and this has still happened; yes I've made sure I'm first selecting the OpenCore EFI on the initial boot and then selecting the installed drive.

The thing is, I have a few other MBP7,1 models and none of them have had this issue with the exact same process. So I'm both confused by that and by the fact that this is not happening either all of the time or none of the time, but seemingly I can only properly boot without that error if I first do a PRAM reset.

Edit to add: after selecting EFI > Macintosh HD > the black background with the white Apple logo and progress bar will load to about halfway, and then the prohibited symbol appears. So it's not just immediately showing that symbol.

Any ideas?