I'm thinking It could be NTSC/PAL format incompatibility. I'm not sure what Russia uses but in Europe, they use PAL. And in the U.S. NTSC is the standard format, but there are some DVD players that suppot both formats.
But the person who created the DVD's might have made them in different formats. If they're bootleg DVD's, the sources might have been different for the different movies.
Last thing I can think of is Region Code, Each area of the world has different region codes for DVD's which restricts them from being played somewhere else.
The easiest way to find out is pop the DVD into your PC & use a program like DVD Decryptor to find out whatever info you can about those DVD's. If you have a DVD burner & some burning software, you can change the region code easily. But if your DVD player doesn't support PAL, then there's no helping that. Hope this helped, Goodluck