The problem could be that you are not watching it closely enough! Just pause it when you are not playing with it.
- To pause a V2 or V3, press button A, then, still holding down A, press B. Do the same to unpause.
- To pause a V1, press B to go into clock mode and then A. Keep holding down A while you press C. If a number is flashing in the date, the pause was sucessful. Press C to uspause.
Getting a Mimitchi is hard work!
I had one once. You have to be on an even generation (2, 4, 6 etc) and take very, very good care of it. Chances are you won't get it first go - it took me over a year!
Always remember to turn out the lights when your Tamagotchi falls asleep, it really helps.
StarTama