Why doesn't my replacement kitchen sprayer work as expected?
I am not sure what you mean when you say you "took the body apart", pictures may help.

Residential kitchen faucets with separate hand-held sprayers (as opposed to those where the sprayer is integral to the spout) do not charge the sprayers with system pressure until you trigger the sprayer. 

They utilize a special diverter mechanism, usually located in the main valve body. The mechanism is a valve that only actuates and diverts water to the sprayer when the trigger/button on the sprayer is pressed. You need to replace the diverter valve.

The below illustration depicts one type of Grohe faucet that uses a diverter valve, there are very likely others: