Have you noticed that your goals are not being updated by the goal rollup job that runs regularly? The first thing you should do is check that the job is not failing. You can do this by navigating to Settings > System Jobs. Then choose "All System Jobs" from the View drop-down and filter to see jobs that have failed. If it is failing, investigate further to determine why.
If you're like me, then you're still scratching your head because the job has not failed yet the goals aren't being updated. I had created several (288 actually!) monthly goals beginning in January 2015 and clicking the "Recalculate" button to update them manually was not appealing at all.
I checked my System Settings to find that the rollup was set to stop after 30 days from the end date of the goal. Aha! This is why all of my goals from earlier in the year were not updating.