Rigidbody w/ sphere collider falls or pushes through mesh collider.

I've been on and off working on a little marble rolling game as a school project, with the main character as a frog inside a glass marble, building the terrain out of blender meshed islands, bridges and fences. Problem is, the two-object main character keeps acting funky with the terrain models. Either both the marble and the frog fall through the island model, or the frog falls through w$$anonymous$$le the marble gets stuck, jitters up and down three times and then falls through, starting into a roll when it clips the tapered bottom of the island - to me it looks like the marble forces itself through the island.

The frog has no collider, the marble has a sphere collider and both exist inside a colliderless empty object for ease of adding. The islands, fences and bridges all have mesh colliders, but the issue only really happens with islands, the bridges and fences work as they should with only minimal blips that I t$$anonymous$$nk I can resolve once I get the movement script up and running.