The length of coaxial cable should make NO difference on any antenna. If it does it means you have a common-mode current issue. That can be minimized by using a 1:1 current balun at the feed point.
If the feed line impedance doesn't match the load impedance than the length of coax will
make a difference even if there is no common mode current.
Since this is a 50 ohm feed line feeding a supposed 50 ohm antenna, I'd suggest that the feed point impedance is no longer 50 ohms. It's unlikely that the coax is no longer 50 ohms but it's a remote possibility.
As others have mentioned, the matching box at the base of the antenna is probably compromised.