Greenhouse Base

I have a 10 x 8 greenhouse that has what I would describe a pre cast concrete base elements they are triangular with the apex cut off onto which the greenhouse base plates are bolted. My problem which I am having trouble solving is that when I lay them and check for level all seems to be ok until the last one which seems to be about 1/2 inch higher and I am buggered if I can work out why.

Any ideas on a solution would be greatly appreciated.

Hi have you any pics of the end profile I think i know the type of base you have ,2 sides are short 1 long iscocelese triangle shape with a flat top I had same problem last year just put a piece of 1/2inch timber on top of all the rest and screwed it down it's now leve,l  ground was uneven   jezza