Try nailing on a bit of that non-slip draw liner, the grippy stuff you can get in rolls.
When we first got our chickens they also refused to use the ramp.. They would all sit at the door with their faces poking out, but none would actually walk on it. They would sometimes fly in and out, but it was difficult and stressful for them.
I thought they were just getting used to new surroundings, but when I watched closely, I saw they were unsteady on the ramp (despite having many little ridges nailed in for grip). They would slip and slide and trip over.. I never knew chickens could be so clumsy!
First I used a staple gun and stapled clumps of straw all over the ramp. They loved the grip it gave them and immediately started using it! But over time they would peck out bits of the straw and leave the staples exposed. To avoid them hurting their feet I went for a bit of old towel instead. I just attached it firmly over the ramp (with the ridges/steps still attached).
They use it with no problem now, but if I take it off, they again refuse to step on the ramp! I just hose it down every once in a while and if it gets too pooey I rip it off and attached a new piece of towel or the non-slip draw liner material. Anything to give them a bit of grip.