I have an RC Guys Decathlon and have no trouble on our grass field. Your picture appears to show the tail wheel not directly in-line with the rudder. I think you should try to elongate the front or rear hole in the tail wheel bracket and get the tail wheel centered. You may have to elongate both holes to get enough play to correct it. Also as soon as airspeed bleeds off, up elevator helps a lot. Hope you get it corrected=it's a nice flying bird
Frank