When I turn on water in the tub, the shower comes on, too. Is this a problem w/ the diverter in the spout?

The diverter is in the spout of the tub faucet. I tried soaking it in vinegar, but that didn't seem to help, except to stop the flow of water from the spout when pulled up. Just replaced the showerhead a week ago and the problem has gotten worse since then. Any ideas? Thanks.