Fixing bookshelf onto plasterboard covered thermalite wall?

[token]

Help, please! I had two shelfs put up in my kitchen a couple of years ago to hold my wife's cookery books. One is OK but the lower started to slope a year ago. I used longer fixings but the problem has returned. I have now tried a variety of fixings (including the ones for cellular blocks and frame fixings) but the holes have not got too wide for anything! I thought I may try to use a resin fixing to fill the holes but the suggestion seems to be to insert a post which can then be finished off with a nut fixing. The brackets I have to use to match the other shelf (and other fittings) require two countersunk screws (1x8mm, 1x6mm) and there is no room to use a nut on the finised side.
Is an option to use a thick piece of dowel secured by the resin fix, that I could then screw into? Any other suggestions? I have not used resin before but it seems to be an idea for getting a good fixing for TV screens, satelite dishes & boilers so thought it may be appropriate here.
The total weight of the books is between 20-30kg, probably closer to 25kg. There are three brackets supporting the shelf, which is just over a metre long, each with a weight tolerence of 30kg, but they are the sort where they are close to the wall and the shelf extends out from them.
And no, one option is not to get rid of the cooking books! Suggested that and still suffering the consequences!!

[welsh brickie]

Try using stardrive frame fixings 100mm do not pre drill use a impact cordless driver or a good drill and drive the screw home it will bite into the block without the need of any rawplug.

[token]

Thanks. I will give it a try