I don't know the Bresser 5-in-1 system, but an obvious question is whether you have a bird guard around the rain gauge catchment. Until I did, I had pigeons roosting on the lip of my rain gauge and it only takes a couple of birds to block the funnel completely. Leaves, other organic material and dirt can also blow into the funnel blocking it.
I have also had the pigeon poop run down into the bucket and solidify so that the bucket is held in a tipped position. Water could run into the bucket and straight out as the bucket was always tipped over. If the pigeon dropping are hard, then it takes a bit to dissolve them.
No matter what, I don't thing that you can get around taking it down and looking. After checking it is clean and free-tipping, pour some water into the funnel and watching what happens.