Baby not drinking mothers milk. This?

My baby is 4 months old.She is breastfeed regularly.For past 15 days she is not latching on by herself when hungry she refuses to latch on unless she is half asleep and sometimes we are forced to make her sleep (really difficult when she is not ready to sleep and it takes nearly 1 hr to 1 and half hrs)to feed her as she is really hungry.mostly during night time i feed her in sleep 3 to 4 times.during day time we have tried leaving her to play and cry hard when hungry so that she might latch on when feed but even then she refuses and turns her head away and cries even more harder..Now recently for past 1 week she has developed a habit of chewing her thumb finger and refuses to have milk even in sleep she just chews her thumb whenever hungry and starts crying and refuses to latch on when thumb is pulled and feed. even when fed through spoon she is not willing to swallow easily. Very fussy. how do we handle this?