Successfully fitting mounting bracket and TV is dependent on two main factors.
1. have you got the right TV bracket try
Beware of cheap import items on the internet
2. What kind of wall are you fixing to
a. brick good fixing
b. plaster good fixing
c. concrete good fixing but hard to drill
d. plasterboard can be a problem, not ideal for large heavy screens. sometimes you may be lucky to find a timber upright in the right position behind the plasterboard.
e. plaster and lathe not recommended unless a timber wall plate is fitted as above