My doctor only prescribed me 2.5mg of focalin (dexmethylphenidate) once a day. is this supposed to be a placebo effect or what?

Not necessarily.: People respond quite differently to stimulants. It's not unreasonable to start at a low dose to see what happens, good or bad. Since they work as soon as you take them, if you don't see any benefits of side effects in a day or two, reasonable to ask to go to higher dose. Typical Focalin (dexmethylphenidate) doses are 5-20 mg/day. You're taking immediate-release; the extended-release lasts 10-12 hours.