Okay these could very well be your problems:
1. There is too much dust on the infa-red thing
2. You are not connecting with the right version (ex. you have a v4 and you want to play with a v2 but you select v4)
3. You are not pressing B when it says stand by.
4. It is a fake
5. It is a baby- babies can't connect...well on some versions
6. You are not choosing the correct option (ex. you select work to connect)
7. You are not holding the tamagotchi at another tamagotchi
8. It is asleep
9. The wind (If you are outside) is blowing the infared all over the place (it's happened to me)
10.You are no going to the heart to connect.
11.The battery is close to being dead