calibre viewer only shows cover page

I am using calibre 4.18 but since the last couple of updates 4.16/4.17 when i try to open books in calibre viewer it only loads the cover page. Dont know what has happened, I have not changed any of the preferences etc. I dont use it to read but to check formats of books or for a quick look at before i download to kindle to re-read. I prefer mobi format but have tried converting to different formats but makes no difference. If anyone knows what's going on can you please help. :help: Many thanks: