joining PVC to CI

I have CI coming up from the floor, and I'm trying to join the PVC main stack to the CI pipe. I've tried using no hub couplings, but the joint keeps leaking around the CI, and the bands keep slipping down the coupling toward the CI. I'm not sure if the CI is slightly too small for the coupling. Is there something I can use to increase the thickness of the CI, perhaps that silicone tape material? I have considered a rubber fernco, but after reading on here that appears to be against code. Any suggestions?

Is the part coming out of the floor a hub, rather than a straight piece of pipe?

Pictures are a plus.

If the cast iron pipe measures 4-1/8" OD, then the proper no-hub coupling will be a Copper x Cast Iron transition.

Some old cast iron is the outside dimension of copper pipe.

I'll see if I can get a photo. Cast comes out of the slab, then there is a hub, and about 6" of CI before I'm trying to make my joint. I talked to someone from Fernco, and they suggested it might be service cast? and suggested using a flexible coupling, perhaps using the metal sleve from the proflex around the flexible coupling if it will fit and I want to be extra cautious. The diameter of the CI is 4.3". It's very close to the same diameter as the pvc