How do I get rid of Fungus gnats in my bathroom? I used a bug bomb but a few hrs later they were back swarming?

the bug bomb only kills the exposed adults, doesn't get to the larvae. Could be bad caulking allowing water/ moisture to get into the walls giving them a place to breed, seal such places will stop them...could be coming from other parts of the home such as nearby plants and are attracted to the bathroom window light. fungus gnats are a tiny fly, where as a drain fly will be quite a bit larger, but much smaller than a house fly, these usually indicate a sewer line issue.