Time flies like the wind, and fruit flies like anaerobic conditions and uncovered food.
If you are seeing fruit flies, or similar flying insects (probably minute scavenger flies), it is a sign that there are conditions favorable for fruit flies in your bin, and they are able to detect it. In other words:
Your bin, or a portion of the material in your bin, has drifted into the area of anaerobic conditions
There is un-buried food material that the fruit flies can easily detect
The steps I'd take:1. Take out the food (pay attention for any of it being stinky, slimy, gooey or just gross in general - it's likely anaerobic and thus attracting the fruit flies)2. Check moisture levels throughout the bin. Correct it where you need to. This will also help keep things aerobic if the bin is too wet.3. Add bedding. This is the fix all of pretty much every problem in the worm bin. You can add it in 2 ways. First you can mix it throughout the bin/problem areas to help correct moisture levels and help keep things aerobic. Second, you can leave a layer of dry bedding (about an inch) over the top of the bin which will help keep fruit flies and other insects from discovering the bin in the first place. Preferably something you can remove pretty easy to access the bin when you need