Make sure it is the adhesive type of Sikaflex.
My guys ensure they don't use it in big blobs on our adhesive jobs, use the traditional builders "W" type pattern so that you get plenty of surface area for it to cure.
We also do not stress for quite a few days, even leaving it a week to fully cure. It can take a long time to fully cure if conditions are not great.
Certainly wouldn't hurt to also score the surface with rough sand paper to help with adhesion.
Normally the problem is getting the glued item back off. The only way of removal is to saw your way through the Sikaflex.
We use Sikaflex to hold large
oil coolers onto body panels.