Edited to add: based on comments. This addition is based on tests that @Greybeard (the question asker) performed.:
It appears that the issue is something to do with the pan itself. The pan appears to only boil on the edges of the pan and may be lacking at least part of an internal layer of the sandwich on the base. Based on this, heat transfer between the element/hob and the pan is faulty, similar to my initial answer (below).
My initial answer was:
It sounds to me like you are having problems transferring heat into the "inner" space of the pot/strainer system.
The most likely situation I can think of is that the inner strainer is impeding the formation of the convection cells in the water within the inner layer. In a conventional pot you would get large convection cells forming between the heated surface (bottom) of the pot and the surface of the liquid above. These allow circulation of the water and rapid heating. With the strainer in place the small holes/solid walls of the strainer are limiting the heat transfer to the rest of the water. It is likely you are getting small convection cells between the pot and strainer, and these are enough that you can heat the water to near boiling, but the lack of heat transfer and with an exposed surface are not enough to get the water to a continuous boil.
Incidentally, I don't think this pot system is intended for pasta, but rather for blanching of vegetables etc, so that you can lift them out quickly and plunge into the cold water. Having said that I don't see a pot/strainer system on my local Ikea site, so I can't say with certainty.