A key aspect of the strategy that we are pursuing is to work out who among the people we meet and talk to is spiritually open. Who is the person where the Spirit is already working, leading them to have a curiosity and spiritual hunger to discover?
A Biblical example of someone very open is Cornelius in Acts 10: a Roman centurion who is already fervently praying and seeking ... so much so that God arranges to fetch Peter to come and preach directly to his household. Lydia in Acts 16 is another. Of course not everyone is already seeking as clearly as Cornelius, and some maybe don't even know that deep down they are searching - perhaps they will only realise much later.
A clear principle we adhere to is to only invite people to some kind of Discovery Bible Study when we sense that a person is spiritually open and potentially interested enough that they might sit down with us. That is based on us gauging the conversations and level of interest (praying of course in the background!). For those that do say "yes" to our invitation, we then find that the preliminary sessions give indications of a person's inner hunger. The simple test is in scheduling the next session - the more open & hungry the person is, the easier it will be to schedule and hold to it. Of course it is not an exact science - things happen, arrangements fall foul of circumstances and so on.
The people who are open to spiritual conversations from time to time, but not really so hungry to explore Jesus, tend to be the ones who let discovery sessions be missed and seem to be hard to follow up. Again there is no exact calculation: circumstances, personality types, and a host of other factors can be a part. We should also pinch ourselves and remember that for a person it may simply not be the right time for them at the moment. Perhaps there will be a different link in the chain of witness that will stir their hunger later.
These observations should feed into our spiritual discernment - discerning who Jesus is calling us to concentrate our own personal finite resources on. We cannot pray intensively for everyone we ever meet, and our limited time & energy means we cannot continue to the nth degree trying to meet up with someone who in reality is not in an open & hungry place to want to meet up. Put another way: it can be okay to let someone drop off our personal prayer list, and it could be the right thing to do to not follow up someone yet again to re-schedule. This can be coupled with an ongoing trust in the Spirit: if the Spirit is intending you to be part of their journey, He will arrange something - be it a meeting out of the blue, the person messaging you, etc.