I’d call that a significant drop off. I don’t know enough to guide you in the right direction but from my stats, last year was the sunniest since 2015 and you’re showing a 50% drop. That suggests to me that’s it’s more than dirt but I could be wrong. I think I would call in expert to check both panels and inverter. If your inverter (I assume it’s only one) has a digital display, see if that gives any clues. There must be more than group of panels going in.I have 16 x 250wh panels, which was the most that the FiT allowed (4Kwh) at the time. There is space for 2 more, but there is also a vent pipe in the way. I am also constrained by the fact that 10 are on a SE facing roof, with a house directly to the SE, and 6 on the SW side, of a two story house. Access to them for cleaning would involving a lot of scaffolding and it looks like the cost of that is probably more than the additional power generated.
Looking at the Kwh generated I am getting :
Apr 2012 - Mar 2013 : 3480
Apr 2013 - Mar 2014 : 3460
Apr 2014 - Mar 2015 : 3027
Apr 2015 - Mar 2016 : 2634
Apr 2016 - Mar 2017 : 2063
Apr 2017 - Mar 2018 : 1752
Apr 2018 - Mar 2019 : 1740
Obviously it is a bit dependent on the weather over the year. I have been told by one of the suppliers I looked at for the Zappi suggested that they do get dirty, even though they are self-cleaning - I can see that on the car even though it does have the Diamond Brite coating on it.
I thought about battery systems in the past but even the Centre for Alternative Technology did not yet recommend it, and the other issue is how well and and for how long will the batteries work.