Straight line machine quilting - stitches uneven

I am trying to straight line machine quilt a child size quilt and I have increasted my stitch lenght to about 3.5 and am using a walking foot. Most of my stitches look ok but some of them are way too close together. can someone tell me what I am doing wrong? Is it the weight of the quilt that is stopping it from moving evenly through the machine at times? I recently had my machine serviced (Bernina Activa 220) so it should be in fine working order for this project. Any advice is appreciated!!! Thanks!

Not an expert but in my experience, sometimes the weight and bulk of a quilt, even a small one, means that there can be variation in how fast the feed dogs move it through the machine.  This can result in uneven stitches.

 have you tried changing the needle
 also try cleaning under the needle plate & in the bobbin area for loose threads & lint

Yes I agree, sometimes a blunt needle can caused uneven stitches.Also try resting the weight of the quilt on a table or chair next to you so that the weight is not pulling the quilt. Could also be that your batting is tot thick for machine quilting.

Thanks for the advice. I ripped all my bad stitches out and am going to try again with a new needle, cleaned machine & a chair to support the quilt's weight. Fingers crossed!