I have been having trouble training with one of the other students at our dojo.
1. Uke is always right.
2. in other cases look at the point number 1
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.
Normally in correct aikido practice the ratio of successful techniques should be not higher than 1 to 50, it means that 49 tries you should fail miserably to get one technique working OK (doesn't mean perfectly of course..). Also using muscles is very good, you have to find a way to use them correctly
Being you, I'd practice as much as possible with this person.