I would not recommend upping your dosage until you know what the problem is. To begin with, your dosage may never have been correct. It may well be too high. Ritalin works great at first, but it will not help you for long if you don't alter the way you do things every day. It will also make you fatigued if you are taking too much over a long period of time. By fatigued I'm talking about a general state, not specific to one particular day. You might have slept fine, 7 hours, just woke up and took your ritalin. You're definitely not tired, but you may still be fatigued. It means that you are generally weakend. Try this: don't take any ritalin one day, and note how you feel right before you go to sleep. If you feel more tired than usual, (specifically, if it feels really GREAT as you drift off to sleep) you are probably fatigued. Reduce your dosage by half and take it twice a day only. Wait at least 5 hours between doses, and eat a full lunch a half an hour before taking the second dose. Do not take any ritalin after 1PM. at 6pm or so, do some sort of aerobic excercise, like running, stairmaster, swimming, etc. for at least 20 minutes. Do this for a week and see what happens.
(NOTE: if you cannot go a day without taking ritalin you have a dependency problem. Don't listen to anyone who tells you this can't happen with ritalin, it can and does. Seek help immediately and do NOT increase your dosage.)
And remember, if you are not making a day plan, WRITTEN, every single morning, and at least attempting to follow it, ritalin will NEVER help you, in any dosage, and may well ruin your life.