I agree with lifting up the quilt in front of the presser foot, so you are "feeding" the quilt to the presser foot from above. It does not take much extra weight to prevent a walking foot from feeding.

Is it hanging up on thick seam convergences? If so, you need to loosen your foot pressure.

Sometimes I use a longer stitch with the walking foot.

Is your test fabric the same thickness (ie two layers of cotton plus the same batting)?

Are you wearing gloves so that your hands have a good "grip" on the fabric and you're not adding extra effort to keep your hands from slipping? If your hands are slipping, that creates 'walking' problems.

What stitch length are you using? Try increasing it . Loosen the foot pressure a tad. Also what size needle are you using? If it's too small (or dull) it'll struggle to keep pace.

You say the fork is on the needle bar. Explain this. The fork should be between the needle bar, not over.

You also might have to lessen the pressure on the foot. On my machines you have a knob directly up from your needle. Less pressure will work better, just turn the knob until the feed dogs can move the fabric along.

I think people don't take this into consideration enough. Several pounds of fabric and batting could be pulling against that little foot and those tiny feed dogs.

Engage your feed dogs. They pull the fabric on the bottom. The Walking foot will help the top fabric. Everything goes thru the machine!

Lots of great suggestions mentioned. Did you have a chance to try any of them yet?