As Lou said, he's seen that extra length of coax make the difference.
That's a good indicator that you need a balun.
The SWR doesn't change along a feedline (except due to line loss.) If you measure a
different SWR when you change the cable length, that's because something changed
at the antenna.
What changed at the antenna in this case is the length of the long wire attached
to one of the elements - namely the shield of the coax, which is part of the antenna.
The purpose of a balun is to eliminate the coax as part of the antenna.