Which Jesus when?

Jesus likens us to sheep and him/God/the Kingdom like a shepherd a couple of times during the gospels.  But as I think of two stories in particular I see two very different illustrations.

"If a man has a hundred sheep, and one of them has gone astray, does he not leave the ninety-nine on the mountains and go in search of the one that went astray?" (Matt 18:12 ESV)

and

"My sheep hear my voice, and I know them, and they follow me." (John 10:27, ESV)

So I see the idea of going out and finding the one who is lost.  Searching high and low for the one that went astray and celebrating when it is found.  But there is also the idea that the sheep know the Master, they follow Him, and so if one strays away does it really know Him.  And is there a point where the Master realises that person wasn't really a sheep and so maybe doesn't go searching?

As a Pastor it's something I am struggling with.  I can see that the kingdom is like the 99 sheep, and it's just not complete because someone is missing. So we strive and search for that one person to come to faith.  But, as a Pastor, do I put as much effort into striving after someone who is clearly ignoring the Master's voice and going off on their own.  Is there a point where the Pastor must focus on those who do listen.

I will always be waiting and willing to share with and talk to anyone who is in need, wants guidance or has faith questions... but I don't think I can pursue someone who doesn't want to be found.