Usually on a tension if the bottom looks loose you have a problem with the upper tension. Try changing it. You might loosen it all the way, sew, then tighten it, sew, tighten it until you get what you want. Did you clean out the bobbin tension spring and make sure that tension is set right? Make sure the correct needle is in right and thread goes the right way. There have been posts on here about setting a tension - check out archaic Annie's post.
https://www.youtube.com/watch?v=kzFX...AyBQ2l-Mzkw7ew she has other info on tensions on her blog.