Leak around Heki, considering the nuclear option....?

[L' Hobo]

I have not read the other responses so may be repeating what others have said. The issue may be the release mould agent they spray on the Heki frame when making it. It leaves an invisible film that will not adhere to Sika Flex even when applying enough to cover the sealing surfaces. I had this problem and resolved it by using a bit of fine sand paper to clean both the motorhome roof contact area and the sealing face of the Heki. Then clean both surfaces with white spirit and allow to dry before applying Sika Flex. Job Done.

There is a number of sprays etc, that can be applied before using the sealant. I use one given to me by a windscreen fitter.