agreed, likely fungus gnats from soil constantly damp
cover plant soil with plastic wrap for awhile, to keep the water out, until they dry out. Then cover plantsoil with landscape cloth, which will allow water in but not allow flies in to lay eggs and repeat the cycle once the soil is damp again. Can also put a two inch layer of rocks ontop of soil, which will again prevent gnats from laying eggs in damp soil.
another alternative is beneficial nemotodes.
Do a search of this forum on "fungus gnat" for other suggestions