Will,
Open the link that Bob posted in another window and locate the "Injector Valve" part # NA0809. Inside that valve is a "jet" - they call it a "Water Nozzle part # NA9809". Normally, after you've tried everything else and your HF still won't draw chems, this Water Nozzle is stopped up. To get it out, you'll have to;
a) take off everything connected to the Injector valve
b) rig the valve so that you can shoot pressure from your TM/portable through the valve BACKWARDS. It takes pressure to shoot the jet/water nozzle out of the Injector valve.
c) Get a bucket. When you pressure up on the valve, you'll need to catch it in something. You don't want to shoot it across the yard so point it into the bucket.
d) Once you've got it all hooked up to run pressure backwards through the valve, and your valve is pointed into the bucket, turn on the pressure. It won't take much and it'll shoot right out.
e) Once the jet/water nozzle shoots into the bucket, hold it up to the light. You should "easily" be able to see right through it. If you can't easily see through it, it is clogged or partially clogged. CAREFULLY use a needle to dislodge the obstruction OR you can replace it for about 12-15 bucks.
I fought with mine for a week before I discovered this interior jet and how to remove and clean it. Good luck. I hope this helps.
James