How can I get my Lervia KH4000 working properly?

Since I bought it, it has only ever done straight stitches no matter what setting I use; also the tension settings don't seem very effective and stitch sizes are the same no matter what number I have it on. When sewing it also misses every few stitches so it looks like this _ _ __ _ ___ instead of _ _ _ _ _ _ . I know I should have taken it back when I first bought it but it's too late now & I have moved overseas. Is there anything I can look at myself to get it working properly?