Flow rolling doesn’t come easily to a lot of BJJ athletes, especially those who’ve just begun training. However, this type of sparring is crucial if you want to develop the feel and the fluidity to your movement; which then translates to leaps and bounds in progress.
The legendary Rickson Gracie recently addressed this problem in one of his YouTube videos, explaining what to do if you want to flow roll properly:

In order for you to flow roll, you have to be comfortable to go from the attempt of a sweep – if he [the training partner] didn’t fall, you should go back into the Armlock. You have to drill this slowly and start to educate yourself.

And every time you go to spar, don’t focus yourself on the results, on the outcome of the fight. Just think about being relaxed, being comfortable… Try to feel what your opponent is trying to do and try to avoid it. Also try to seek for positions that he gives open and go effective on them.

Rickson also emphasized that it will take years to really learn how to flow roll, and that the majority of this skill comes from drilling:

It’s just a flow. You cannot put too much on your mind [to it], expectations, frustrations… Be calm, be comfortable. If you miss [a move] this time, you’re going to get it another time.

Years and years of practice will give you the sense that you don’t have to think about anything, but that you just have to obey the flow. And then take the responsibility from your mind and give the responsibility to your educated reflexes; which have been educated by drilling, by knowing exactly how to grab, how to choke, how to hook, how to sweep…