I bet that most of that 50% you think are just tolerating you are going to look back in a few years and then appreciate what you taught them.
How old are the kids you are teaching? These days I've been doing my best to help my 11 year old niece with her homework, but it's a bit of a struggle even though she is a very, very smart young woman. I do wonder if Facebook and the rest of the instant media has limited her ability to focus on doing her homework..... she just doesn't know how to do the research for herself, I find. But perhaps she's a bit young for me to expect too much quite yet, I'm not sure.
I teach 11-16 (sometimes 18). A big issue over here at the moment is "Independent Learning" and the kids inability to do it. We are working on ways to improve that situation.
11 is a good age here - we get them from primary school at 11 and my current Year 7 group is lovely with mostly bright-ish but nice kids who are prepared to work and prepared to learn. The trick is keeping them like that as the hormones kick in! I have a couple of extremely bright classes, too, aged 12-13 and 13-14.
They are my reason to work at the moment...I love the challenge of stretching them, of managing to give them something mathematical to marvel at that doesn't appear on their TV screens!
I admit to failure where the kids are less bright, and less well behaved. There are members of staff in my department who are excellent at this, whereas i am not. my forte is with the higher achievers, where my colleagues often can't cope. Horses for courses, as they say.
With your neice, the only thing to do is to keep at it...if the attention span is short, keep the sessions short...I often have 2 or 3 different activities in a 50 minute lesson. Have the niece explain to YOU what she is doing rather than you explain to her. She may well know more than she thinks, and explaining may draw that out...
And focus on basic skills...in the old days we used to chant our times tables, we did our calculations on paper. If this stuff becomes second nature as it is to me, then the harder/different things she will be doing can take precedence. One of the barriers to our teaching all the kids need to pass their exams is that they can't do the basics. no point teaching the solving of quadratics to someone who can't add two 3-digit numbers together!