Hi,
Like you said, flies are seasonally abundant in some regions. While you do a root cause analysis, ensure that you thoroughly scrutinize your sanitation and hygiene before looking at those beyond your control.
The first point is your kitchen area. From your statement the focus is on the canteen area. Ensure that your kitchen has window and door screens, no left overs,decaying and open waste. The flies could come in through the backdoor and enter the kitchen then from there proceed to the canteen through the connecting door. This said, sanitation and pest control in the kitchen and storage area is very important.
Incoming raw materials should be checked properly. Some raw food supplies can introduce flies into your premises. Make sure they are well packaged, no signs of decaying parts and supply trucks should be hygienic too.
Another area is the toilet/restrooms.If they are not well situated, sanitized and insect proof they could cause flies to get into the canteen. These should be cleaned regularly and sprayed with non irritant insecticide when necessary. Check the drains for leaks and vents should have screens too.
The canteen area which is your major area of concern requires several strategies. Just like the kitchen, the tables and other surfaces should be cleaned regularly. Left overs should be evacuated promptly. All the windows and doors should have screens and they should be checked for holes once in a while. Insectocutors should be installed like you have done( though not above eating tables), then non irritant insecticide should be sprayed at night when there are no customers. Then for the entrance, self closing doors are the best( the simplest is by the use of springs). Like I've mentioned before, if you have the means you can also use powerful air curtains to keep the flies out. One strategy used where I worked before was a door man/security. The doorman(extra salary) opens and closes the door when the customers come in and when they leave.
You have little control outside your immediate premises. Ensure that your waste bins are lidded and are kept far away as possible from your food preparation and service areas. Ensure that your wastes are evacuated promptly and also spray insecticides there. If you discover that the flies are coming from a neighbour with their waste bin close to your fence, there's little you can do other than to prevent entry to flies from there. If it's an understanding neighbour they can listen to you and relocate their waste or store them appropriately if you complain to them. You can also volunteer to provide an appropriate waste bin for your neighbour( I doubt if they'll accept though).