UV Holes

Im have a major problem texture mapping, every time i texture map a model and apply the texture map i get uv “holes” and my texturemap never pertains to the mesh the its uvmapped- unless i auto unwrap which does no good when im highpoly model, since im designing levels and etc. i need full control of uv placement as well as tiling. Anyone got any answers or ways to texture an entire level with tiling? As well as the problem of uv holes. Thanx for any help