I don't know a huge amount about plumbing / toilets, so I'm hoping
someone can help me with this.

Recently the toilet cistern has not been filling up. This is easily
resolved by simply unscrewing and lifting the cistern lid, and then it
fills up without me having to do anything else. It's an intermittent
problem, but it's a pain having to unscrew and lift the lid before I
can flush it! The toilet I have is a Bellavista Stylo push flush.

Can anyone help explain to me what the problem is and how I can
prevent it?

Thanks.