The trailing edge margin is bigger when printing on cut sheets than it is with roll media. The manual gives the margins for sheets as 5mm top, left and bottom, and 17mm on the trailing edge... 17+5=22mm - about the 3/4" you're missing, assuming you're printing a 30" image without scaling. Cut sheets require this extra margin to give the pinch rollers that feed the paper something to hold on to at the end of the print, a restriction that roll media doesn't have. Unless you can use rolls, you'll have to scale the image down to 29 1/8" on the long side to print the whole thing.
Jim