If HP inks are dye based rather than pigment based, you will do better if you use a mordant of your fabric first - Bubble Jet is the one that comes to mind. This helps the inks to stick to the fabric and become more colourfast(washable).
Another trick I have used for pieces smaller than A4, was to run a piece of paper through and lightly print (draft) what I wanted to print, then to put double sided sticky tape along the lead edge & sides of motif. Cut the fabric to fit to the outer edges of the sticky tape, place it down and make sure that it is firmly in position. You can get away with a bit of overhang on the trailing edge, but the lead edge needs to be smoothed as much as possible, then you can run that through your printer. This can be reusable for at least a dozen times. I have also come across the idea of sticking the fabric down with ordinary sellotape
Small advert - my friend Ruth Brown at
http://stonecreeksilk.co.uk/has a good book on 'Digital Imagery on fabric' - where I got most of my useful tips from, and has some interesting information about different printers & how thing work. I even got a couple of my pieces photographed for the book too