If someone can't make you move, see if you can figure out why, and then help them figure out how to make you move - or - ask the sensei something like - "I don't think I'm trying to stop my partner but it's happening - can you see if I'm doing something to interfere with the technique or if there's some error my partner's making?
You've mentioned that you progressed pretty quickly but not what your rank is - however if you've only been in Aikido for less than a few years, you're in a situation of not knowing how much you don't know.
(I've 'passed' a nidan test and I still get very confused at least once a practice, either with what's being taught, why it's working so well, or why it's not working at all, and I try to figure it out whatever the case)
If your partner or potential bully is getting frustrated because you're harder to do a technique with, help him (assuming a him) learn how to do the technique or movement principle better so that you can learn more about a) how to do it better yourself and b) how to read the movements of your partners for practicing kaeshi-waza
Becoming an "expert" or "master" takes (on average) 10,000 hours of practice. Studies are showing that there's a range from about 6000 hours (makes me jealous) and infinity (person should give up and try something else) - maybe you're one of the "early" types.. or maybe not.
Not sure that's exactly what I'm trying to say, but if, in the future, it turns out that this fellow is actually 'bullying', then do it Alic's way...
It seems like everyone can take me down just fine once sensei reminds them to use more weight underside. I keep trying to tell this student this but I think it just makes him worse, as I'm below him in rank.
I'm been at it for 7 months and have passed two tests, my second one with a total of less than 6 months in. Right now I'm at 4th kyu, preparing for 3rd.
Thank you so much for the advice! I appreciate the time taken to comment.