I second getting bf specialist advice.
Unfortunately many m/w and h/v just do not have the expertise, and if they do they don't have the time to impart it.
So generally the “solution” to any breastfeeding issue is formula top ups.
For example not gaining enough weight and you’re told to top up with formula. That often leads to a downward spiral where more and more formula is given, as mums get worried about weight and with formula you have an absolute measure of how much they’ve had.
I was lucky enough to have a hv who supported bf. He- yes strangely enough the best bf advice I had was from a male hv- firstly was against weighing, especially weekly as some hv like to do with bf babies. He went on clinical signs instead- did the baby look healthy, hydrated, good ins and outs, alert level etc. Said weight was just a meaningless number without the rest.
When mine did plateau a little weight wise I got lots of advice on increasing supply, what I ate etc, and also tips and tricks on how to make them take more. I also got lots of reassurance that she may just be a small active baby who burned a lot of calories, and all this was purely precautionary as she looked completely healthy.
Also I was referred immediately to paeds, for blood tests and assessment. He actually discouraged formula top ups as he said babies nearly always gain with formula, but that wasn’t necessarily a good thing as if there is an underlying issue that may mask it. So get the paeds referral done first so you know there isn’t a liver, kidney, thyroid or other issue that is causing an issue.
When she passed all the paeds tests I stopped getting her weighed and that made things much easier. It gave me the confidence to know if she was happy and active she was doing well just on breast milk.
Just a different perspective.
O/p if your baby has lost weight then I’d push for a paeds referral. Weight gain slowing on bf is normal, but weight loss is a different thing. Get any biological reasons ruled out.