These days I just use a third party service to sustain these claims for me. Not worth the time, effort and anxiety in dealing with these companies. I used AirHelp (they're a german company) and received compensation from a bunch of Wizzair flights. Airhelp has an easy online form which always works, you can upload documents, screenshots, etc. And they keep you posted with periodic emails on the outcome. This is not meant as an ad for them, I'm genuinely satisfied even though they kept some money as part of their commission fee.