Reply to Problems with the beatgrid in Traktor

hi! I'm an amateur dj from Chile (South-America).
I've been into djing for like 4 months and using mostly Traktor Pro 2.

The thing is that I was looking for some star-ups for Traktor and one of them was setting a correct beatgrid for your songs. I've learned the basics how to do it, but suddenly I was noticing that in a few songs the beatgrid was correct and the beginning of the song and, as you go along, the beatgrid was not that accurate. The beatgrid was starting moving to the right or left of the beat assigned.

I was believeing that this was cause because of the setting the wrong BPM for the song, but I'm not really sure.
So that is why I'm asking.
If you guys could help I would really appreciate it.