4mo DD is almost EBF except for one formula bottle of 3oz or so at bedtime before her breastfeed.
Feeding seemed to be going fine up until recently, she started to take less time feeding in the last month or so but I read that that's normal as they get more efficient.
Unfortunately in the last few days, maybe a week?, she has started pulling off the breast and crying. I usually offer feeds every couple of hours or so in the day / on demand. Previously she seemed happy to feed at these times but now, at most feeds, she sucks for a while and it seems that if she doesn't get a let down sharpish, she starts grizzling/grunting while on the boob and then pulls off and screams at me. I try relatching her again and again but same happens. If I do eventually get a let down she then seems to feed OK - but not for very long.
I should say that I have never felt let down, but I can tell it's happened by her sucking changing and the sounds - swallowing, pausing for breath etc. Can hear that there's milk there.
It seems to me like my let down is sometimes taking too long and she is getting frustrated? At one feed yesterday it never seemed to come. Am getting quite upset and stressed which I know won't help. Her sleep has been awful since Christmas so I'm exhausted and sleep deprived too - could that have caused this issue?
I'm trying to persist with each feed even through the screaming until I hear what sounds like a let down / until it sounds and looks like she's getting some milk, but not always possible.
It's affecting her naps too I think as she's started fighting them more and I think it's because she's hungry. 
Not sure what to do. Worried my milk supply will reduce because she's not feeding so well.
She has been hovering around 25th percentile since birth but when I weighted her most recently she was 22nd and hadn't gained anything since her 3rd round of jabs on New Year's Eve.
Any similar experiences or words of advice?