At those times it may be useful to spend some time cultivating joy. When there is joy in the mind, the mind will be bright and not as dull. This is partly why the suttas say joy is a support for concentration, and not only a result. While I agree with Ben that we shouldn't crave for pleasant feelings, it's to me quite obvious the Buddha did describe the results of meditation as pleasant. And one of these results is joy. If this doesn't arise, I think it's good to wonder why. That's not necessarily craving or aversion.
Specific meditation subjects I find very useful to create some joy are contemplation of virtue, of dana, and contemplation of the (characteristics of the) Buddha. Loving kindness can also create joy, but it is a bit more indirect. All those meditation subjects are also mentioned in the suttas in one way or the other, so it's not something exotic I am advising here.
We had a thread lately that put some emphasis on them also. There I also said that for me breath meditation can be enough to create joy on its own. But it is not always like that and then those other contemplations can assist.
It's impossible to say exactly what causes what in other people's meditation, especially over the internet, so see the above in this light. Although the above will most certainly help in a way, there may be another issues beneath it for you. For example, when I am in a period of general sense indulgence, I am more likely to experience similar meditations that don't really take off and are dullish or bland. So to decrease your sensual intake may be another thing to consider. Also the amount of rest/sleep I find can have quite an influence.
Hope it helps in any way!