O.K...Sit down a minute and let's talk...there...cup of tea? I've been there too.
... some kids just have some trouble getting a grip on math and some of its concepts.
First, I suggest reading The Three Barriers to Study And How To Overcome Them - a great little booklet that helps you *and* your kids understand what happens to make learning a chore *and* how to get back in the swing. I printed it out, punched the margins and tied it with a shoelace. They enjoyed reading it themselves and could sure relate to what is said. I learned a lot about how they learn and how I must teach from it too.
I read a lot of Valder Learning Systems stuff on teaching math, and went back to square one with them, as she suggested, then we worked on making sure that the basic concepts were grasped, with no onus at all if it took a while (I had to learn to leave my exasperated face in the closet for a while 'til I didn't need it so much anymore...LOL).
Now, I use the Mathmatics Worksheet Factory (you can download their Lite version as freeware from Softseek.com) to print out daily practice sheets (note - "practice" seems a lot more palatable to them than "study" here), and Key Curriculum Press Key to Algebra and Fractions workbooks.
There is very little whining now, from them or me... 8^)