If you’re not getting the response you want from your InMails, the problem could be as simple as your subject line.
With LinkedIn again tightening up the consequence for InMails that go ignored, improving your response rate is not only good for your sourcing, but good for your wallet. You don’t want to get burned by LinkedIn for getting negative feedback on InMails, and you certainly don’t want to repel potential candidates by reaching out to them in ways they don’t appreciate.