Plumbing Questions? Ask a Plumbing Expert.
I can link you to the troubleshooting guide for this type of issue. You'll need a multi-meter to troubleshoot this. The troubleshooting is too involved for me to tell you what to do here. It would take forever. Or are you just looking for some likely causes so you don't get hosed by a repair tech? Let me know if you want the guide. I can answer follow up questions if you have any while following the guide.
I just noticed your model number. It looks like this model has a standing pilot so the service bulletin I was referring to doesn't apply here. The most common reason for the unit to not respond is a clogged inlet filter on the cold water inlet. Have you cleaned this filter/screen?
Cleaning the filter is the first thing you need to do whether you think the flow is good or not. It's also the easiest thing to do
I'm not going to be on much longer. If you don't get back to me in time just relist the question and one of our West Coast experts should pick it up. Or I can get back to you in the morning ~8 am EDT
If cleaning the filter doesn't work and you've tried cleaning the flame sensor (as described in the manual) then we're down to the water valve (not the shut offs). Thw water valve is what senses the water flow and modulates the burner flame accordingly. It is a maintenance item. Here is the service bulletin for rebuilding ithttp://www.bosch-climate.us/files/201109161614460.CT-23_Water_valve_rebuild.pdf
You can also move on to item 7 in the manual, checking the voltage at the electro valve
Then you need to test the voltage as described above and possibly rebuild the water valve.