Bath tub not molded correctly?

While renovating a bath room, we added a bathtub/shower. After completion we noticed that some water does not drain. It collects in the middle of the tub. It's not a lot of water, but still should not be there. The contractor says that the tub was not molded properly. Not sure if I believe him since we had some other problems. Of course the wallls are all in and finished now, so my question is whther there is anything I can do about this. If I have to go into the wall and try to shim under this area I will, but is there any other suggestions.

First thing to check is with a level. Place it on the edges and see if the thing is level both from side to side and from end to end. If it is, then yes, seems like the tub may be defective. If it isn't, then it was not installed properly. The rim needs to be level for any tub to drain properly.

Even money you will find the tub is not level. A very slight out of level condition could cause your problem.