Gee, that sounds familliar! That used to happen alot to me when I was starting out. We were riding cheap Barum tires and we used to have the basetapes separate all the time. The product that we used to use was Barge cement. I think it was for installing carpets, or something. It worked, anyway. The trick was to make sure that the surfaces were clean and dry. If you were trying to repair early-season tubulars, the road grit made it almost impossible to get a good bond.
Solving this kind of problem leaves most people without easy access to Barge cement in Europe clueless.
They either have to resort to rim cement which is a bad idea or find a shop that offers liquid latex and take a shot at it.
If there are places that still carry latex based carpet layer's glue then that would do as well but those are far and inbetween...
So, when based in Europe, what to do?
P.S. Before panic breaks out, having to reattach base tape is normally only of concern to those who actually repair their tubs by hand (thread and needle jobbers so to speak), base tape needs to be reattached after sewing or when using el cheapo tubs that have not been properly built in the first place and which you'd really not want to use anyways.