Slate expanding in the sun - Slate paving

[orsonmca30]

i have recently installed a slate patio and it seems to be expanding and lifting off the the sharp sand bedding mix i have layed them opon in the sun shine.
the mix i used was 5 parts sand to 1 part cement and the sub base is 9 inches off compacted mot type 1 road scalpings.
the joints are 10 mm wide and filled with a strong motar with a added cement dye (black).
has anyone heard of this happening before and have i done anything wrong, what can i do to fix it.the customer is not very happy........

[andpartington]

was the beding wet or dry and what mix did you use a pick would help

andy