I pull on them to check them. If I feel a little give, I undo or cut them and retie because I know my knots don't come undone. If they seem strong enough I leave them.
If you contact the company or yarn shop over just one or two knots, you may not get a satisfactory response as many will say that up to 3 knots in one skein is 'industry standard' or normal. If you have much more than that, or find thin or weak spots ro the color fading in spots, contact the company and they often send a replacement or a coupon. This happens in cheap acrylic or expensive 'name' yarn from the LYS. Usually the 'cheap' yarn has less, at least that's what I've found.
sue- knitting heretic