To build on what Sarah said - you can also look within channel if you have access to someone with the technical chops.

Some ad groups or campaigns could be driving "worst" leads and others could be driving "best" ones. To cut off the channel is throwing the baby out with the bathwater.

UTM tags in adwords (or any other platform these days) will be key to figuring this out. If you get leads on the phone, there are awesome solutions out there as well.