Some pages may require more toner (at a faster rate) than others. If the toner is running low, sometimes there is still enough for text but not enough for heavier coverage like graphics (and this can often happen in part of the cartridge first). Shaking the cartridge might help more evenly distribute the toner in the cartridge to temporarily solve the problem. Just be careful as with some cartridges, that can cause quite a mess. We go through several toner cartridges at work, and its common for one side or area to go first when the cartridge is low.