This is an obvious one, but I'll mention it anyway. There are two different DVD formats, DVD-R and DVD+R, newer DVD burners will handle both, but older ones often only record one and not recognize the other--I have pulled my hair for a fair amount of time before realizing that I was using the wrong one--

I have also tried to burn DVD's in a CD player--and once I bought a whole spindle of DVDs that turned out to be coming apart at the edges. That kept me going for quite a while. Good luck!