Ok... I have one of those situations were there is no error and it all works (but just 10% of the time...)
I uploaded to sharepoint Online a simple xlsx file with just a simple 3x3 table with random stuff. Then I add the excel web access part, select the excel and it ok in the properties. And by magic the worksheet is showed correctly. Then i change webpage (or hit refresh) and I got a blank web part... if I edit it sometimes it shows the excel in edit mode but if I click on save it stops working again... if I make changes in the excel sometimes the excel shows up again some times it doesn't...
Some hidden parameter of feature that might be wrong or something? I'm trying to find something related to access times to the server (cloud) or something like that but I cant find anything...
Many thanks for you time guys :) pleaseee help ;)