the knobs probably broke. Very common fault. New ones are much more durable. If you were to remove it and have a look inside you'll see the plastic shaft broken, the new ones have a brass shaft.
There's usually spare ones knocking around on ebay
here's the complete kit http://cgi.ebay.co.uk/Glowworm-CXI-...14&_trkparms=72:1683|66:2|65:12|39:1|240:1318
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