Turf wars is fickle.
Suggest:
SWITCH to easier weapon (ink brush is not bad but it is hard weapon to use in turf wars, range is small, doesn't spread as much ink as a splattershot/airspay or roller). Switch gear to add more ink saver main or swim speed up. Or try a different weapon with a more special weapon focus that throws ink for turf wars, tenta missiles, rain cloud, bomb rush. Switch your gear to add a special weapon charge up gear. People are mentioning weapon choice because it makes a big difference.
CHANGE your play style. Move to center using different path. Try inking only and running from fights where you haven't taken opponent by surprise. Try running to your mates or switch it up and go where your mates aren't. Try never using sub weapon (not effective use of ink for spreading ink).
AWARE of map. When respawn or at quiet time, look at map. Look where enemy ink is growing, go there, or go where enemy isn't.
LAST 60 SECS check for ink on your side of map, go clean it. Actually go squid mode and hit map. Most of your mates will fight middle or push ahead can cover more turf if split up. Try not to die, dieing in last 60 secs is bad, it's usually far to get to where you can be useful. Actually run away from a fight. If your totally advanced at last 60 sec if your mates are alive and near middle of map push ahead, if dead or recently dead, find an area without enemies to ink, even if on enemy side and avoid fights because you have to ink.
SAVE SPECIAL WEAPON for last 60 sec if it throws a lot of ink, bomb rush, rain cloud etc. dump in large area.
Remember don't get down turf war is fickle.