Welcome. What are the lines like without pressing shift. I only ask, because if 'ortho snap' (which constrains a line to only x or y direction) is turned on, whether you are using snaps or not. Then the shift key over-rides this. So if ortho is on, then shift will temporarily turn it off, and vice-versa. If ortho off, shift creates straight line, ortho on, shift can create angled line.
I'm Sorry if this sounds basic, but I don't know your expertise with TC.