50% isn’t bad.
Remember, you’re connecting to strangers on a “professional” social network.
Have you experimented with no message at all?
It might be work doing 20-30 of those to get a base line number just in case the actual message isn’t the issue. i.e. you get similar results with/without message.
In that case it could just be that lots of your prospects just don’t sign into LinkedIn very often.
