There are many possible reasons for your sprayer not pressurizing. If the pump lever is moving freely and does not seem to be building pressure while pumping it is possible that the O-ring at the valve is damaged, the seal in the pressure regulator is leaking, the piston or the collar is worn. Any of these problems will require disassembly of the unit and inspection of each part. If you do feel resistance building as you pump but quickly loose pressure you likely have leaks in the diaphram, it's seal, the pressure cylinder or the tank itself.
If you are comfortable disassembling the unit and inspecting the parts for wear or damage most of the individual replacement parts are fairly easy to replace and readily available online from Solo or at many retail outlets and supply companies. Solo has done a great job with how-to videos and instructions for each procedure on their web site www.solousa.com
If you are not comfortable disassembling and repairing your sprayer yourself you can try contacting a mower/equipment repair shop in the Redding area as is seems the nearest Authorized Solo sprayer repair center is Cloverdale Saw in Cloverdale CA, 707-894-2273