Why is my baby being difficult at feeding time at 5 months?

she adjusted to the bottle very easily at 14 weeks, but since shes turned 5 months old she is very messy when i give her the bottle only taking 4 oz at a time instead of the recommended 7oz. she has only been like this the last two weeks, she is in good form, sleeping well and alert, so i cant figure out why she has suddenly gone off ingesting anything more than 4 oz at a time, its like she gets bored...im just worried about her not getting her daily quota of milk...any advice?