Well dude, first of all, I'm soooo sooo sorry that you have to come to our sorry ass website to ask for help.
As far as your question is concerned, you said that you try to tackle with Monty, but I get the feeling that you aren't referring to the slide tackle technique, but the hit, that is performed by shaking the Wiimote. In this case, I'd tell you that you have to keep in mind that Monty Mole, or any character, will hit the nearest character to them when you swing the remote.
I'm willing to bet that you might be getting a little frustrated, and swinging your hits out a bit wildly, and that's why you aren't getting the desired result. Just try to relax and work on the timing of your hit, and you should eventually be able to direct the attack in the direction of the character you intend. Monty may be the most effective hitter in the game, and your time and practice will pay off dividends.