Several times throughout Act 1 and Act 2 of the movie, characters, especially those in the First Order, make major decisions based on getting BB-8 because the droid has the map data. But why would they assume with pretty much complete certainty that BB-8 still has the map? BB-8 could have either given the map up freely or had it forcibly removed (we see that the droid can be captured very easily). The map could have been passed on from BB-8 at several different points while still on Jakku, it could have been passed along in space around the time that Han Solo and Chewie arrive, it could have gone in any different number of directions from Maz Kanata's planet, and so forth.
True, Kylo Ren lets BB-8 go when he thinks he can get the info from Rey, who has seen the map. But even then, they're thinking that BB-8 still has the map, they just don't need it anymore. (And later, Kylo Ren sort of gets admonished for not going straight after the droid.)
Related problems are that the First Order could have accidently incinerated BB-8 and the map when they attacked on Jakku, and since the Resistance seems to have spies in quite a lot of places (and definitely at Maz Kanata's), it actually would have been much smarter for them to have BB-8 pass the map on to someone else who had not already been identified and tracked by the First Order. (For example, they could have sent off a few ships in different directions from Maz Kanata's, with only one having the map, but the First Order would have to pursue all of them.)
Here are my full discussions about The Force Awakens with Jason Book. The first video focuses a bit more on positive aspects of the film, while the second video delves into a series of criticisms.