Hi Dan
It is a lack of adequate flow, not adequate pressure (although they can be related) that prevents the valve from indexing properly. We make an electronic monitor for these valves which confirms that they are indexing evenly and are not getting stuck due to debris in the valve, broken spring, etc. In general we find these valves are very reliable and work well as long as they are being operated within their intended design parameters. KRain say a minimum of 10 gpm is necessary for the valve to index, although we have found that as little as 6 or 7 gpm will usually work.
If you have lower pressure than you used to, it may be due to a break in a line somewhere that is allowing leakage out of the system. Other things to check: Has the spring inside the stem broken so that the rubber disk it is not going down all the way when the pump runs? Take the top off the valve, and if the spring is good, the rubber disk and stem assembly should pop right up. We have a short video on our web site for the valve monitor which shows taking the top of the valve and the rubber disk popping up. It is located here:
http://dynamicmonitors.com/videos/
See the video called "IVM6000 Quick Start Step 1 - Replacing the RDSA"
Try and push the rubber disk down into the valve with your hand, to see if there is any excessive resistance that could indicate a broken spring. Will the disk go down all the way easily? You can get a replacement stem to try for probably less than $20 if you are unsure. KRain also offer lighter gage springs that allow the disk to go down with less pressure/flow. So if you decide to get a new stem to try, you could ask for one with the light duty spring.
Also check the cam located under the top of the valve cap in the center which could have become worn, or there may be debris lodged in there that is preventing the stem from moving down into place. You could try replacing the cam if you are unsure, as it is a very cheap part. Any KRain distributor should be able to get them or have them in stock.
Hope some of this helps. Let me know how you get along!
Kelly