The Epson P800 has three official paper feeds (and one un-official variation):
Most of our customers are using the second, Front Fine Art feeder most of the time. And most of the time, if you follow the instructions in the PDF manual that came with your printer, things will proceed very smoothly.
Here's the manual again, if you happened to have missed it - in particular it has useful visual guides to using each of the three paper feeds:
Whilst the feeds work the intended way most of the time, life is not always neat and pretty. And fine art papers, whilst all looking like similar things, are actually quite variable - in weight, rigidity, and flexibility. And some are more tricky than others for printers to feed reliably, especially as you move up in sheet sizes towards A2, by which time the sheet is very large and heavy in paper terms (and expensive, in human terms!).
So - here are a few possible methods to load these papers should you run into issues (like paper skew errors, failure to load the media, the rear tray slamming shut etc).
Here's how you do it - and note at no point should you need to to use any substantial force, you should only meet minor resistance at most during this process:
This approach uses some basic bond/reflex paper to basically make a temporary ramp for the paper to slide up.
With any luck one of these methods gets you happily and consistently printing, even with difficult medias!