Matt your quite right about grass cuttings (particularly when they are fresh and green), it also might be to do with the closed system your using. The plastic bins you get from the council generate lots of heath and sweating. One thing worms don?t like is heat or extreme cold. Each species has a tolerance temperature rang where they are happy and thrive, and likewise a food source they prefer.
If grass cuttings are to be added they?re best put in when they are well rotted on a standard compost heap and past the heat generation stage.
Open systems are, George tells me, the best for composting and worm breeding. The website the Monk makes reference to has open systems for sale as does
http://www.nurturingnature.co.uk/
In my open system I put all organic waste including cardboard, paper, teabags, food, garden wastes. In return for feeding the worms I get a rich compost which is one of the best growing mediums possible.