If there is even the tiniest bit of thread or lint caught in the bobbin area, that will mess up your tension. It may be necessary to look at the bobbin area (without the bobbin) with a magnifying glass and good light.
Also, thread or lint caught between the upper tension discs will mess up tension. Try lifting the presser foot (opens the tension discs) and "flossing" between the tension discs with *unwaxed* dental floss or several strands of thread held between your hands like floss.
Examine the bobbin. Is it tightly wound and not over-filled? If the bobbin has been wound with insufficient tension on the thread, the winding will be loose and that also can mess up tension. Also, make sure the bobbin is exactly round. A distorted bobbin is bad for tension. Plastic bobbins are more likely to warp than metal bobbins, but metal can warp too.