I don't see why it matters if the uke is junior. i don't think the OP said that. It's interesting and symptomatic that uke is being blamed for nage's technique not working. There's a good chance uke is feeling muscled and just not responding to being pushed around. The OP might want to ask. Atemi goes both ways. If nage feels free to toss in the odd atemi so should uke. Same for judo and other extra stuff.
It sounds like this was static practice. Some folks can make static techniques work without forcing things. Most people (me especially included) can't, in my experience. Resorting to tricks is more about winning than about practicing. When my nages do that I give up and fall. They are signaling they don't want to practice anymore and I don't care if I lose when I'm not competing.
I would willingly take the blame the uke responsing to my muscling technique if I thought that were the all of it, but even our shodans have problems with this person. The only person who doesn't seem to have any issues is Sensei - but he is 8th dan and doesn't have problems with anyone.
I haven't gotten the habit of muscling completely conquered yet, but I have made improvements in leaps and bounds. I know the Uke was deliberately using strength however because I have an incredibly smooth (almost sneaky) shihonage, and I felt him reach up with his other hand and deliberately attempt to reverse it and dump me on my head. I disengaged rather than one of us get hurt at that point.
This is why I question his temperment and don't entirely trust it. I don't mean to assess blame (even tho I know that is what I am doing), I just am stating the situation how I saw it, and will plan on not getting into this type of situation in the future. Much like you said above...I will just go with it, and as Maggie said earlier..I will learn from it and leave it up to him if he chooses to do the same or not.