So, I'm helping out a friend (note, his wife just fired his PB a week ago and ripped him a new one - the pool was a mess)...
In the last week, his Jacuzzi "stopped working". The spa jets have come to a trickle, and the blowers don't send air to the jets anymore... but all of the equipment is working.
I've verified the pump is working (it's a 3HP WhisperFlow), yet very little water makes it to the jacuzzi. I've checked valves A, B, C and D on the drawing, and they are all working and in the right position. I've closed C and left D open (as well as the opposite of that) with almost no change in flow to the Jacuzzi. Water coming from A into the pump has no problem (I surprised myself because the pump is below the jacuzzi, water came out of the pump from A's direction - with a vengance, but very little from the B direction).
I find it highly unlikely that both lines C and D are plugged west of the C and D valves (or more accurately, west of the two blowers as air from them is not making it to the spa either), but can come to no other conclusion - and find it very coincidental that it happened within the last week or so.