If it it only happens sometimes, then its possible that it is not the stim med., but your lifestyle (not getting enough sleep for example). The effects of the medication should be consistent - not change one day to the next (unless, you are also on another medication that might interact with it).
Essentially, the effects should last a max of 4 hours. Any longer would indicate an overdose. For example, if the tiredness only happens after the 2nd pill - then this would be a real possibility.
You need to document when you get tired as that will help your doc. Its possible that you need to take your second dose later in the day or reduce it.
Either way, you should let your doc know what is going on. By the way, your doc should have started you on the lowest dose, not the 10mgs.