Magnavox msd804 vcr/dvd combo

I think what you are referring to is the pinch roller. This roller sits at the right side of the mechanism, and in play mode, presses the tape up against the capstan shaft. These pull the tape through the machine at a precise regulated speed. If the roller isn't being engaged or the capstan isn't turning, then these conditions need to be corrected. Does the capstan turn freely? How about the noted roller? Perhaps lubricant has dried/hardened/gotten sticky...

There is electronic circuitry for the capstan motor (a direct drive type motor integrated on the underneath side. Something could be awry with this circuitry.

There is a drive clutch mechanism which could also be defective. This drive clutch assembly sits in between & swivels between the supply and takeup spindle, depending on whether VCR is in Play/FFWD or REW. In the Play/FFWD, it is engaging the right spindle (takeup) and in REW the left spindle (supply). If this is what you are describing, perhaps this clutch unit needs replacement or the grease at its pivot is hardened/sticky.

Most VCRs of the last 10+ years don't use a rubber tired drive (very common on late '80s VCRs), rather a gear toothed drive.

Beyond a simple mechanical problem, repair might not be economical feasible. Most VCRs of the last 10+ years are poor quality, disposable, unlike the machines of the '80s and early '90s, which were built like tanks comparitively speaking.