Please tell me how to make a breaker fit tight on the buss bar?

Are you certain you have the right breaker for the panel?

I am using what Lowe's told me to use... they are fitting loosley and causing them to burn where they are connected to the buss bar

Get the brand and model number off the circuit breaker panel and purchase the correct breaker. Do not accept a sales clerk's word.

Remove one of the circuit breakers that fits properly and take to store to match.

I don't want to be an alarmist and you are probably all right purchaseing from Lowe's, but be aware that there are bogus circuit breakers out there.

Google "bogus circuit breakers".

You are not putting it back in the spot that burned, are you??

That spot is no longer usable AT ALL!