This has probably been stated over and over again, but if you want to continue having conventions, then make the number of fans needed a very easy achievable number. I haven't seen a successfull Convention in UK1 or NU1 in over 2 weeks and most of the Cons haven't even got to 50% of the goal.
Everyone is so frustrated with the non-stop FAILED Conventions, that no one even attempts them anymore. Either tweak the totals to a realistic number or remove Conventions all together.