My babies not holding necks up😔

Se
This is making me sleepless. My twins will turn 4 months in 4 days but still can't hold their necks up. When I put them against my chest, they try balancing and rest after a minute. And then hold up again but they aren't steady. They were born at 35+2. They hate tummy time and start crying after a minute. What should I do? Please help!!