Drain Flies in shower

In one of my washrooms we have flies that keep flying out from the drain after we take a shower I have tried bicarbonate soda and white vinegar for five days and nothing changed. Please advise what can I do this is a nuisance and we are unable to use the shower.

Drain flies (also known as humpback flies and Phorid flies) lay eggs in drains. The eggs hatch and the larva feed on scum in the pipes.   To eliminate them, the entire interior of the pipe must be treated with a foaming insecticide.  This will contact all the eggs and larva.  It is important to never let the water in the p-trap evaporate and dry out.  A well equipped pest professional will have a foaming machine and the right product to take care of this.