Ah! Yes. I experienced this when I started. here are some trouble shooters:
- Turn off the GTS and the turn it back on, this can sometiems reset it.
- First make sure the Pokemon you are searching for is not illegal (i.e. a lv. 9 and under legend)
- Your search may be set to some remote place like the Bahamas. So make sure your search location is everywhere( or if you are looking for a Pokemon of a different nationality search Japan)
- Make sure you are not looking for a high level 1st evolution Pokemon. (i.e. lv.100 squitle, cause more often than not, there isn't one)
These are the most common errors made, if it still doesn't work, it could be that you have a faulty system.