Respected ayatollah khamenai has given the fatwa that tatbeer is forbidden, also respected ayatollah sistani has given a fatwa that states tatbeer is forbidden whereas some other ayatollahs state that its fine to practice tatbeer
My question is:
1) Is tatbeer jayaz or not jayaz if yes/no then why?
2) Why do all the ayatollahs give different fatwa on a same topic?
3) If the one whom I follow states that tatbeer is forbidden I should follow him but the other says no it isn't forbidden then the people who follow them are wrong?

by

1- S. Sistani has not given any fatwa, neither positive, nor negative in this regard. Hence, we can not say it is jayez or not jayez.
Since he do not have a statement about it, you can refer to someone else, and follow his fatwa.
2- This thing was not practiced in the Imams days. Hence, there are no narrations about it. Therefore, no one can say it is haram.
Saying it is haram depends on certain criteria, such as:
1- If it leads to bad image of Shia.
2- If it leads to harm your body seriously.
3- If it harms someone else, like those who are practicing it with their kids.