I have had this problem on a few occasions. Usually I got ignored over suggestions on points the lead author did not want to discuss directly because (i) s(he) felt (s)he was on the right side; (ii) s(he) felt (s)he was on the wrong side but wanted to push their version nonetheless; (iii) s(he) felt I was not in a position to criticise that part.
In all cases I demanded for an explanation, and this is why I can answer your question here. So, my first suggestion is that you contact this person demanding an explanation. (Mind that most people don't do these actions -- questioning parts of manuscripts where they are co-authors, and later seek to understand why they got ignored -- and that most lead authors prefer having passive "co-authors".)
How did I react in such situations described above? I will briefly explain:
(i) I tried to make my point clearer, which subtly meant I was trying to prove the lead author wrong. The lead author always insisted on their point without seeming to listen to what I said, while emphasising on how they took my suggestions on other points. I had to let these issues go, and they got published, and as a result I do not fully agree with some of the papers I participated in.
(ii) I insisted. This will make the lead author uncomfortable, and you will have to make sure this person understands you're not cornering him/her. Typically there is a clear mismatch with data or logic that the lead author doesn't want to hear aloud. Usually I got the problem fixed, usually the lead author wasn't happy and is now unlikely to collaborate with me in the future. I think that is OK, given the circumstances. Once I had to let it go.
(iii) This is the most common situation. Typically it involves a collaborator from a different field under strong influence of their bossy PI, where you're not sure which of the two is doing the writing or even answering to your emails. They do not appreciate being questioned "on their turf". You have to consider the possibility you're wrong there. Which I think is irrelevant, because if you are wrong then there's all the more reason you deserve a clear, logical answer. In such cases I insisted, and was authoritatively told to "keep to my business". I insisted again, and got ignored, and had to let it go. Usually the lead author apparently did see my point but was told by their PI (i.e. the last author) what to do. As a result the lead authors in these cases made sure to communicate they were open to collaborate with me in the future, while their boss cut communication.
Anyway, hope that helps. In the long run you're selecting whom you can work with.