I was at a composting workshop over the weekend and we were checking out someone's wormfarm when the topic of worms escaping came up.
The woman who was taking the workshop (a rather respected and knowledgable gardener round these parts), said to add some lime or wood ash when this happens to adjust the PH balance. Perhaps add less acidic material too?
Tom, Flo is right about the PH balance if the WF gets too acidic then the worms don't seem to like it and you need to bring it back to the alkaline side of the scale.
I came across the folks from Earthly Delights at a market day who explained the PH balance in a WF to me. You can get the a lime mix made up for your WF from them for just a few dollars and you don't need too much in a small home set up. Check out the attached link for their contact details.http://www.earthlydelight.co.nz/