Mamiya M645j film counter not advancing to start when roll loaded

You should be able to get replacement light seal kits on eBay for most cameras, but they may need a little judicious trimming for a perfect fit.

If the problem is on the magazine's counter and if the spacing of frames is correct then the counter dial is loose. two screws on the outside front of the top of magazine, two screws in the film chamber hold the top cover. remove leather cover on winding lever loosen screw and remove. you can then remove the complete top housing to expose the counter mechanism.

Mamiya RB67 Pro SD that is new to me. loaded the film correctly took the first shot. But, the film won't advance. What am I doing wrong?

From the manual:
"If the error display is not canceled when the shutter release button is pressed again, insert the Dark slide, half-press the shutter release button while pressing the roll film holder's manual film advance button to wind up the film, then contact your nearest Mamiya agent or service center."

If you need a manual, you can download one from
http://butkus.org/chinon/mamiya/mamiya_645af/mamiya_645af.htm

A few things to consider:
-Loading: Make sure you are not over loading the film before you start to shoot. I have sometimes turned the film advance too many times and end up cutting down on the amount of usable film.
-Are you loading the film in a dark area and careful to close up the back just after it has started to load to avoid exposing it?
-Shooting: Are you metering properly when shooting? Obviously underexposed images won't develop. You should consider a hand held meter.
-Shooting: Are you advancing the film more than it needs to be after you take your shot? Watch the film counter and feel for that click as it hits the next frame. I have advanced past the next shot many times by accident.
Hope that helps, Matt

There is good chance that the back needs a spring replaced. I've seen them fail many times.

This is not the best solution as it indicates a faulty film back. The film should advance without resorting to activating the film-wind release lever. That is only for intentionally advancing a partially exposed roll of film. Make sure the multi-exposure lever is not engaged and that the counter does advance and the red mark disappears as you advance to an unexposed frame.

film not advancing? film not rewinding? film transport motor dead/ needs replaced.
Rewinding but counter not resetting and light meter dead? Prolly bad connector or ic inside camera = repair shop.

Perhaps you've already solved the mystery. There are 2 parts to the back of that camera, the film back + the part that it slips onto. And they have little safety latches/pins that couple in order to advance the film & release the shutter. I'm sure those just aren't in the right place. Here in Los Angeles you could take it to any major camera store or repair shop & they've help you out.