Push Button Toilet not filling up?

I have recently had a push button toilet installed. It has only been installed for a month or two. It has always filled up fine after flushing. It has recently started taking a very long time to fill up. I have looked around for some advice on how to deal with this and tried many thing but I still dont know what to do. Here's where I am:

I have isolated the water to the cistern by turning the isolating valve and emptied the cistern.
I then took out everything I could, cleaned them the best I could by dismantling them and put them back.

Still no joy...

It sounds as if the water is coming into the cistern and going straight out again. But its not going in to the toilet through the flush. I don't know a lot about toilets especially the newer push button type so any help would be appreciated.

Many thanks