Try putting a few strands of steel wool in the hole to give the screw something to bite on.
This seems strange but works like a magic charm .
I pulled a few strands of 0000 steel wool from a pad , evenly spaced them into the hole with a chop stick and carefully screwed the bolt back in place ...make sure all the washers are in the right places and put a drop of synthetic 2-stroke oil under the sprue plate to lubricate where they rub when opening and closing .
My bolt loosened every other cast , put strands of steel wool in hole and after several casting sessions it has not loosened ... I put a screwdriver on the bolt but it didn't want to easily un-screw , I was going to clean the block top , when the bolt held tight I left it alone ... that's been about 4 years ago ...!!!
Try it ... very easy , quick and cheap fix and if it doesn't work ... Oh Well , nothing Lost.
Gary
PS - use the synthetic 2-stroke oil as mould lube , apply with Q-tip sparingly , a tiny bottle will last a lifetime ... good stuff , better than beeswax !
The steel and aluminum like to gall so keep lubricated .