My question involves the texture of my bathtub.?

I want to be able to use a bath mat on the floor of my tub. You know, the kind that have those little suction cups. I bought one today, at Walmart, placed it on the tub floor, but, for some reason it will not adhere to the suction cups. Part of my tub is porcelain, but, the floor of the tub has a bumpy surface. I suppose for the purpose of safety. But, it does not clean up so easily as the rest of the tub. What can you suggest. I hope Im saying this right, so that you understand my message. Please, can someone help me with this? Thank you.