The questions were asked, “What do you make of the statement that Jesus made after His resurrection in Mark 16:17-18?” and “Why do you think that these things are not done by most of us, do we not believe or is it simply a lack of practice?”.
I think these are exactly the signs provided by at least some of the disciples (many in Acts 2, Peter in Acts 3:1-10 and Paul in Acts 14:8-10, Acts 16:18, and Acts 28:3-6 come to mind). It may be these signs were only for that time and place. However, I think it more likely that these signs are needed where ever the Holy Spirit needs to validate His message and messangers.
But this does not mean everyone will do these things. I say this because Paul was quite clear that each of us have different gifts (Romans 12:5-8). I think that when/where God needs to manifest specific gifts for the growth or edification of the church, He does so.
As to why do most of us not manifest these signs? I don’t think most of us need to do so. We each have different gifts and different callings (Romans 12:3-8; 1 Corinthians 12). Not everyone has the gift of healing. Not everyone speaks in tongues. Not everyone interprets.
But if the question is, “Why do not more of us manifest these signs?”, then I think the answer might be different. It may be that God has not thought there was a need for these gifts to be manifested. It may be there is a lack of faith in the believer and God is limiting Himself to our faith levels. It may be people’s faith has been damaged by the church or by charlatans posing as believers from God and then being exposed for what they are. It may be the philosophy of the world that “magical thinking is for barbarians” has invaded the church. It may be disobedience on the parts of some. But ultimately, I think it is because the Holy Spirit has not decided to give these gifts to more people. It is GOD who decides where, when, and who these gifts shall be given (1 Corinthians 12:11).
But just because someone manifests signs does not mean they are from God. Christ Himself said there would be many who would claim to have done miracles in His name and He will say He never knew them (Matthew 7:21-23).
Before worrying about signs and miracles, be sure you are right with God. Be sure your motivation is right. Desire the more spiritual gifts that edify the church, rather than the ones that edify yourself or are intended for unbelievers (1 Corinthians 14:22).