Lightmapping Issues

Hi there!! I'm developing a car simulator, and it's been a pain getting the lightmapping right, one of the first issues, is that some very small props don't proyect shadows properly onto long surfaces, and not as clears as other light maps protected by other bigger models. Does it have to do with UV size?

The other very important issue I ran into is that every time I tried to lightmap my scene, it disables the normal maps I previously had in all my models, and although I have them active, they won't load at all. I'd love to know how to solve this one the most, because my scene is very plain without normal maps

Thank you very much!!