What will be the effect if the threaded portion of conduit is not fully fitted to the coupling?

In the United States, the NEC requires that runs of conduit be continuous and connections be made wrench tight. If done properly this provides added grounding protection to your circuits. If a connection is left loose, that added grounding protection does not exist.

There are times when threaded connections simply won't thread completely. This may require that the conduit threads be "run" with a threader to remove whatever is preventing this from happening. With experience you learn what is acceptable or not acceptable. Generally, if the connection is "wrench tight" and is otherwise secure, that is sufficient.