I agree with what Julie said. There are so many sweepers and so many sweeps. If you keep entering, you will win.
Are you using Roboform? It will save lots of time.
Have you tried varying the time of day you enter? For example, if you always enter the IWs at night, try a totally different time for a week or two, mix things up a bit. It is no guarantee, but worth a try.
Some people say it takes a few months, maybe four, before you start seeing wins. There is some truth in this because it may take months for the sweeps you have been entering to end, select/verify winners, send out prizes.
I guess you have also been entering the Expiring Soon sweeps, especially now at the end of the month.
Sometimes it is good to take a break (days, weeks) and focus on other things. Then when you return, you are refreshed. I also like to read and reply to posts in the forums and vote for fellow SAers (even though I will never enter a voting contest myself). I think it is like good karma, and from time to time amyshulk mentions it too, that when you help out other people, it seems to help you too, (what goes around comes around) so I'm not the only one who thinks this works.
Hopefully your winning will begin to happen soon. Don't give up!
Good luck!