Large format sheets need to be inserted into the film holders in the correct orientation, otherwise you will be attempting to make an image through the anti-halation layers on the back of the film. If your colour film (it's most obvious with chromes) comes out a deep under-exposed red, then it's likely you put the sheets in the holders backwards.
Sheets have notches at one end of a short side. While facing the emulsion side (i.e. while loading the holder), the notch will be:
When you look at a developed sheet, you're actually looking through the back of the film so you'll note that a properly exposed and developed sheet will be in the opposite orientation to the above description when the image is properly oriented.
Many films have the film-type imprinted in the edges of the sheet and this text is oriented so that it is not reversed while viewing the film from the rear. If you can read the text (i.e. it's not mirrored and the notch is in the opposite location to what is described above) but your image is mirrored compared to the scene, your film was installed into the holder backwards.
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