Hi A.,
It could be 1) that you're just not *feeling* the let down anymore. That happened to me after awhile. Some women never feel a let down, as frustrating as it can be.
2) Your pump has lost suction. I used the Evenflo electric pump and mine "worked" for 7 months, but was never really what it should have been. I will be buying a different kind with my upcoming baby, due in November, since the Evenflo wasn't very good for me.
You can try a warm compress or breast massage before you pump. That helps some women. I was pumping every 3 hours as well and dropped to about 1.5 ounces, from both sides combined, after a few months. You can try pumping in the morning/evening before or after you baby wakes up/goes to bed. That should give you some extra supply and the morning session should give you a better result. Also, try oatmeal to increase your supply. It worked for me. There is also an herb called Fenugreek which is known to increase some womens' supply dramatically.
All that being said, the IUD might have some effect on your production. I've never been on birth control/hormones before so I'm not sure. Your OB/GYN might be able to answer that one.
Good luck and congrats on the new baby!
Added: I found this on kellymom.com
Milk supply: For most mothers, progestin-only forms of contraception do not cause problems with milk supply if started after the 6th-8th week postpartum and if given at normal doses. However, there are many reports (most anecdotal but nevertheless worth paying attention to) that some women do experience supply problems with these pills, so if you choose this method you still need to proceed with some caution.