Well I was in the same boat a few days ago.. my LO would not feed properly & i was getting frustrated as i thought that he wasn't taking in enough calories, so i truly understand what you are going through.
My LO is now on solids & has 3 mini meals a day& up until a few days ago his milk intake decreased significantly. I was pretty sure it was due to him being constipated & stopped giving him solids & concentrated on his milk feeds, also offered prune juice. Still following EASY i would let him have how much he wanted & if he didn't take enough during the day, i would give him a cluster feed before bedtime to make up for the missed daytime calories.
My guess was right... 24 hours later he had a big bowel movement & his milk intake went back on track!!! It was such a relief!!! The weird thing is that during the daytime, if i offer two milk feeds about an hour & half apart, he would not be interested but with the cluster feed he is. Today as an example he took 10oz for his cluster feed, bearing in mind that his usual feed is only 5-6oz.
I agree with Paula not to bother going with the age recommendation on the packs... we use tommee tippee closer to nature bottles & DS was on teat size 2 at about 8 weeks & size 3 at about 5months.
Before i changed teat size, i noticed that my DS, who is a fast eater, was taking longer to drink his milk & wouldn't finish his feed, therefore takingless calories.
Hope that helps.Let us know how you get on.
P.S. Paula thanks for your help!
Naz