Kitchen sprayer only uses half the water, how to fix?
Sounds like it is a problem with the diverter. It could be clogged, rusted, or even damaged, and as such, may either need to be cleaned (soaking in vinegar may help), or outright replaced.

There are a couple types that Moen seems to use, so which one depends on your model. There is a collar-type vale:



And there is another type with a smaller inner valve:



You'll have to open the faucet to get at it (how you do this again depends on the model), and then pull out the diverter with pliers.