On my iPod 2nd generation, it won't play .mp4 videos that I uploaded to Dropbox. Does anyone know why this is?

Because the format is not compatible with the iPod. There are many variations of ,mp4 format

The 2G iPod will natively play:

H.264 video, up to 1.5 Mbps, 640 by 480 pixels, 30 frames per second, Low-Complexity version of the H.264 Baseline Profile with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats; H.264 video, up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second, Baseline Profile up to Level 3.0 with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats; MPEG-4 video, up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second, Simple Profile with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats

There may be apps that will play your video format

Yes. You need SD resolution (640 by 480 pixels). I have used the free HandBreak computer program.

The easiest way is to just use iTunes on your computer to crate a compatible version. The option is in the Advanced Menu option.

You will need a 480px MP4 video. You can convert it using various programs.

You should convert it to 480p