manual sync problem with DDJ SX.

Hello, I have purchased a ddj sx, I have the latest version on firmware installed and the lastest version on Serato in my macbook pro. I also have analyzed all of my tracks with beatgrids on serato dj and also turned on the quanticize option, however, when I am mixing or looping, I perfectly beatmatch manually my tracks but eventually my tracks seem to unsync slowly on their own... this hasnt happened before when I use other pioneer equipment. Is there something I forgot to do?