It is always difficult to motivate someone. But it is very easy to demotivate a person. So you have to do a lot of work if you want to motivate a member of your team and you may destroy all this work with one reckless act.
Motivation is an own attitude of a person. So you cannot order someone to be motivated. But you may use another fundamental behaviour of mankind. All people will compare themselves with others and if they want or not they will always in a competition with others.
So let’s try the following: concentrate yourself on the motivated members of your team. Praise them for their good work, for example during a team meeting. But on the other hand do never criticize unmotivated people even if they do something wrong. Criticism will always lead to less motivation. So praise good work and don’t criticize bad work.
If you do so you will activate the competition feelings and maybe you will also encourage them to imitate the other team members and do the same good work to get a compliment. The chance to get a compliment is always a stronger motivation factor then the fear of criticism.
By acting in this way you also solve another issue. In normal team you have more motivated people then difficult unmotivated people. If you are focussed on the difficult persons, then you will spend most of your time with only one or a few team members. If you instead focus on the motivated and therefore the bigger part of your team, then you are back in the team. And as a project leader all of your team member will need you, so don’t do the fault and focus yourself on one or two challenging people.