From my perspective, this is not a Six Sigma problem. However, it does seem to be a management problem. The motivational issue will likely disappear with some proactive leadership. The difficulty you are have in relating your issues to the bottom line is likely due to the nature of your accounting system (i.e., a mismatch between the key issues and the chart of accounts). Consequently, you may have to amend, modify, or otherwise augment you accounting system in order to establish financial correlations. Naturally, this is not done overnight.
In the interim, it may be necessary to simply “edict” these projects in lieu of having people voluntarily pick them. Remember, not everything in this world will happen with the word “please.” Some things require “do it.” Good leaders know the difference and when to use one, the other, or both.