Volunteers are motivated by all sorts of things to get them into volunteering. Once they are volunteering with you they can be demotivated by parts of your sport organisation (or disorganisation!) that are not looking after them properly.
To stop your volunteers being unhappy, some of the more basic needs of "human motivation" could be put into place. This idea can link to Abraham Maslow's hierarchy of human need (1954). Often volunteers might be prompted to volunteer because of the higher levels of need like "self-actualisation", but the reasons many of them may leave may be because of the more basic needs in the safety and physiological needs categories.
These basic needs of volunteers include: preparing for your new and current volunteers, managing their time well, making sure they have someone to talk to about problems, and succession planning for your volunteering.
Your organisation should walk the fine line between managing your volunteers so obviously that they feel they are at work, and not managing them at all so they become demotivated. Research from the Institute of Volunteering Research in 2003, A Choice Blend talks about this fine line.
Some of the things that your national organisation can do to put the building blocks of volunteer management in place are:
If you already do all of these things and want to achieve the next step: the gold standard in volunteer management, then your national organisation should register to achieve Investing in Volunteers.