Well, I guess I will keep trying with these two. I figure eventually I will get it right. However, I do know that if there is a bunch of females next to a male, that a preference will show up over a day or two. I have also seen bettas who are just not compatible, in fact with Plakat, to such an extreme that the male will kill the female. Probably my own fault, nonetheless it happens.
Now if a shop tells me these two are a pair, I can only believe that some attraction has been observed, why else should I pay more and rely on the statement that they are a pair?
Actually in this case, the female is bigger than the male and i already noted this before trying to mate them. I did pair her with a much bigger, quite aggresive crowntail. After two spawns she is a lot less stroppy in the spawning tank and i think I will soon get them paired up and spawning together. Just took a bit longer than I first thought.