Solar pv running at quarter power since out of action

[youaremysunshine]

Hi all - solar panels were removed from an outbuilding roof for the roof to be replaced - then the panels were replaced - there was a gap of about 4 months when the panels were removed and the system isolated. Since reactivating the system the reading on the panel is hovering around 2A 500w, with the occasional bump up to 1.5kw - apparently it was running at 1.8kw on a good day previously, and has anyone got any advice for me as to the reasons and a possible solution? My knowledge is partial, so any help is greatly appreciated - thanks!!! :)