Welcome to KnittingHelp!
So annoying. This can be a problem with many yarns, some more than others. You just have to undo or cut out the knot and join the two ends of yarn the way you would join a new skein. There are some tips here
about a third of the way down the page that may help with joining. When it happens three times or more in a skein, I complain to the LYS and the manufacturer.
Which yarn are you using?