How to work with devs as a junior UX designer

I work as a junior UX designer in a small team of a department (consists of 2 junior ux/ui designers and a non-designer team lead). There is no senior designer. The reason why I emphasize on this team structure will be explained shortly.

So as a UX designer, I have a lot of interaction with the devs... especially the FE/BE developers. And whenever I present the designs to the dev team, the team-leads of the dev team always have something to say to go against my design. Which I totally understand, they have their professional opinions while I have mine.

But one time, I presented my designs to the clients in a meeting, and they were satisfied with the design solution that I've came up with.

And then later on, I presented the same design to the team-leads of the dev team, and one of them had strong feelings against it (not all of team-leads on the dev team, just one of them). And when I asked what was his reasoning, he would just say it is 'too complicated' to build it.

Although, I had my heart set in stone that this is the best UX solution that I could give for the product after a lot of research, but I tried to ask the team-lead more questions as to why this is not feasible other than it being 'too complicated', and tried to give my reasoning why this is the solution. But he just did not want to proceed with the discussion.

And the team-lead of my team, who is not a designer, is just there listening and says he has no idea what to say on the design, and wanted me to go back into my 'research-mode' to see if I could find a better solution. I have to go research again while the clients were satisfied, but just because one dev said it's too complicated.

Thus, I guess my questions are...

• Can the devs get away on opposing the design by just saying "the design is too complicated"? (I mean the UX designers have to come up with at least one valid reasoning, imo, to back up a design solution. But the devs's situation doesn't seem to be the same)
• Or is this because of my lack of seniority and experience as a UX designer that cause the devs just to say no to the design?
• Is there anything that I could have done better in this type of situation? (Should I have asserted more on my design with more reasoning?)

I really appreciate your advice.