Do you change the fructose solution, at least once, but preferably twice a week? I feed my mice a 32% fructose solution (D-fructose diluted in tap water and sterile filtered) and I find they have no problem drinking it, but you have to change the solution and the bottles regularly as two things can happen: the solution dries in the spout and blocks it, or that the solution starts to smell bad - the solution has a high concentration of sugar so it shouldn't get contaminated, but I have noticed the smell changes.