Sewing machine, Lervia KH4000?

Hi,

I only used this machine for a couple of short jobs years ago and some months ago I've started to use it regularly, however, I've experienced lots of problems sewing denim as the piece of fabric doesn't move, the zig zag is always the same as the length and width of it is not what it should be no matter how many times I've set it with the right settings.

With light fabric the straight seam is neat sometimes and others not, again no matter how many times I set the settings .