Carpentry and Woodworking - Difficulty with miter saw

I am doing some work on my shed which requires cutting some new 2x4 rafters and braces. I decided to buy a 10" power miter saw to make the cuts easier. I was going along fine ... I had cut 5 rafters ... and numerous braces ... on the 6th rafter I brought the blade up to speed for the 26 degree cut on the 2X4 and the blade would not bite ... I thought there was something wrong with the blade so I bought a new one .... it gave me the same results .... does anyone have an idea why the blade will not bite into the 2X4.... thank,,,,,sorryistart ....

Not really sure what you mean by bite.

Do you mean that the spinning blade makes contact with the wood but does not cut when you apply pressure.
Could the blade be spinning backwards or installed backwards?

Like above, not sure what you mean... but - check for scraps from your last cut that might be keeping your motor/blade assembly from lowering completely to the cutting table. Without the wood in place, does it move up and down freely?

And/or is the blade nut loose, allowing the blade to stop when pressure is applied?

You hit the nail on the head! ... the blade/motor assembly was not lowering completely (you'd think that I would have noticed, but I took a leap to blaming the blade and didn't look back) .... the saw is equipped, unbeknownst to me, with a STOP LATCH which is to be used when moving the saw ... its purpose is to secure the motor/blade assembly in the lowered position ... but I must have inadvertantly pushed it in while working and if its pushed in while the blade/motor assembly is in the up position, it prevents it from lowering completely .... so the whole afternoon was shot while I ran to Loews to get a new blade ... and then to the hardware store (multiple times) to get a screw extracter and a new screw after ruining the screw that holds the blade hood in place and needs to be loosened in order to get to the arbor ... and then after finally replacing the blade getting the same problem ... I was pretty frustrated ... but thanks to you I have regained my sanity and eventhough my shed roof isn't done (and snow is on the way) I at least have a spare blade, a screw extractor, some helpful experience and an excuse to stay home from work to finish the job... thanks again ..... ,,,, istarted ....

Glad it worked out - now - don't forget where you stash the extra blade... cause the day is going to come when you need it "right now".....