I just picked up a yellow/blue polarizer cokin filter, and when I attach it to my 70/300 the lens cannot find focus....it works fine on all of my other lenses.......
The polarizer eliminates around 2 stops of light so there may not be sufficient light for the AF detectors. If your lens is an f/5.6, with the polarizer on it will now transmit light as if it was f/11 which is not enough for most AF systems.
You don't mention which body you are using, but it may simply be a case of not enough light getting through to the AF sensors. The 70-300 is comparatively dark in the first place (especially at the long end). Add a couple of more stops of darkness with the filter and problems may surface. It's all theoretical without knowing the details, but I think I'm on the right track. With your camera and the 70-300, do you ever have trouble focusing in low light?
I've encountered the same thing in almost every AF camera I've owned from the 8008 to our current Fuji S2's and D2X's. It's one of the reasons we still hang onto our vairous D-series f/2.8 AF lenses, even with DX lenses as our mains. That extra couple of stops wider aperture can make a huge difference in low light, especially with filters.
I'm not telling you that you have to scrap your 70-300 and pick up an f/2.8 zoom if you hope to use the new filter. Instead, you can use a few "stunts." Ideally pick a more contrasty point in the view at the focus distance you want, focus there and reframe. You might also gain ground by zooming wide in order to increase your min aperture, focus, then rezoom to the framing you want, compose and shoot. If you're working from a tripod, it's also possible to remove the filter, focus, turn off the AF, reinstall the filter and shoot. I prefer to use the "sheet" style of this filter (for Cokin holders) rather than a screw-in, because it allows me to easily slide it up out of the way for critical focusing, then slide back down for the shot- even when handholding. And last resort with my aging eyes, I even shut off the AF entirely and resort to manual focusing when circumstances dictate that I leave the filter in place and none of my other tricks work.
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I'm not telling you that you have to scrap your 70-300 and pick up an f/2.8 zoom if you hope to use the new filter. Instead, you can use a few "stunts." Ideally pick a more contrasty point in the view at the focus distance you want, focus there and reframe. You might also gain ground by zooming wide in order to increase your min aperture, focus, then rezoom to the framing you want, compose and shoot. If you're working from a tripod, it's also possible to remove the filter, focus, turn off the AF, reinstall the filter and shoot. I prefer to use the "sheet" style of this filter (for Cokin holders) rather than a screw-in, because it allows me to easily slide it up out of the way for critical focusing, then slide back down for the shot- even when handholding. And last resort with my aging eyes, I even shut off the AF entirely and resort to manual focusing when circumstances dictate that I leave the filter in place and none of my other tricks work.