Of course some people will always find a million cheap excuses to justify their poor technique, don't be misled.
Your partner is very right to introduce ‘difficult' attacks; his job is to guide you out of your comfort zone. This is only way only changes can be done in your body, because such situation force you to find new solutions, and consequently it means a jump to higher level of understanding aikido.
Being you, I'd practice as much as possible with this person.
I don't think the OP is looking for excuses but exploring the situation to better understand what is going on.
That said, I agree that it is important for uke to offer more resistance as directed by the instructor. Aikido training has do be done in a cooperative manner lest someone get injured. An uke who after being asked to "lighten up" but refuses isn't helping nage learn anything but how to be a bully. If the uke is not offering some guidance, some kind of direction as to how nage can overcome the additional resistance then the uke is not helping and is only creating a hostile training enviornment where again someone could become injured. Based on what the OP stated, there being issues of the same nature with other students, I would be inclined to beleive that the uke is doing a disservice to his/her training partner.
I like working with folks that will challenge the level of my Aikido. Working with those folks is frustrating but a tremendous pleasure all the same. But there is a difference in challenging someone to perform better and being a bully on the mat.