I'm adding circuits to my panel, but the homeline breakers I'm putting in ( a homeline panel) won't snap in all the way. I'm afraid to push really hard to avoid breaking something

Pull out one of the old breakers and be sure they match perfectly.

Do you mean the outboard side of the breaker won't lock into place?

Are you using tandem/twin breakers? If so then the spaces you are trying to install them in will not accept them. Some panels will not accept ANY twin breakers.

http://www.foxelectricsupply.com/Content/Media/Images/Product/hom2020.jpg