Not intending to contradict, but it's unavoidable... Higher resistance would make the battery last LONGER, but give a poorer hit. Are you using LR atties? That could explain it. Also, batteries get old and don't hold a charge as well. How old are the batts?
FYI - I use a mod...and an LR atty. Gave up on the standard cig-size 510's after my initial few weeks. A DIY box mod with a 14500 battery (900 mah) is inexpensive (under $10.00 DIY, under $30.00 purchased assembled). A DIY MadVapes tube is what I'm using now. The batteries are < $5.00 each. I think the charger was about $10.00. I have 4 batts. They last me 1/2 - 3/4 of a day per charge... each. It's like a mega ego in capacity at $5.00 per battery.