In the November 2011 issue of Muscular Development my trainer Hany Rambod was asked the question about not being able to get a pump all the time. He said that water wasn't the issue since he drinks at least a gallon a day. Here is what Hany had to say.
"There could be a few possible culprits at work here. One such possibility is that you may have been using a nitric oxide product for so long that your body has down regulated its response to it. The body will become acclimated to arginine and citrulline based products at some point, and their effectiveness is greatly diminished. the solution is to take four to six weeks off from using before resuming. Another reason is can be impossible to sustain a pump is inadequate sodium. This could either be due to intentional dietary restriction, or the use of any number of medications that cause sodium depletion in the body. Antibiotics, blood pressure meds,and anti-estrogens are just a few that can elicit this effect. Water, sodium and carbohydrates are all needed to facilitate a pump and maintain it." - Hany Rambod
So if your having trouble getting a pump go through the check list and also make sure your getting an adequate amount of rest.