Dual flush toilet doesn't dual flush

[Mike_B]

I have a Grohe dual flush toilet and the flush button looks like this:

I'm assuming that pushing the smaller inner button should trigger a small flush and pressing the larger button should trigger a large flush. But whatever I press, the toilet always flushes the same way. I think it's a big flush but I'm not sure. I measured it and it's about 15 liters each flush.

Any ideas what could be wrong and if it's an easy fix that I could attempt to do myself, or is there no option but to call a plumber?