question about presser feet

I have noticed some peculiar behaviors with my sewing machines, it seems that if the nose of the presser feet is not at the correct angle while its on the fabric the stitches will not lock in much if at all. this even happens on my vintage machine where the tension disks engaging are not dependent on whether the presser foot lever is up or down. on some thick fabrics i have to push down hard on the nose end of the presser foot to get the stitches to lock in correctly.