Really sucks after a few levels. Since you can only proceed if you manage to get rid of the animals right in front of you ... what, if it just doesn't match? Within 4 minutes? No progress at all.
How about an bonus system which prevents the game to get stuck? Each time 20 of a kind (in total) get wiped away, the player can destroy one free animal ... something like this. Or after getting a combo of 5 animals at once, 3 random animals of the same kind get destroyed too. Not as efficient as the previous one, but at least something. Guess/hope you understand what i mean.