Issues with Quicktime Pro playback on web

Hi everyone,

I compressed some videos using Quicktime Pro 7 and gave them to my webmaster and they seem to play on some computers but not all... People will click on the thumbnail that contains the videos, but nothing happens, while for some people, the video just pops up and plays. Here are the specs that I compressed with:

Video Settings:

Compression: H.264
Data Rate: Restricted to 500 - 1200 kbits/sec (varies from video to video)
Optimized for: Streaming

Size: 720x486
De-interlaced

Audio Settings:

Format: IMA 4:1
Rate: 22.050 khz

Prepare for Internet Streaming is checked with a fast start.

What can I do to make these videos compatible for everyone? People who cannot view the videos can go to Youtube and view videos there. Is using quicktime to compress the videos the issue? Is it because I am using Quicktime 7 pro and others use a lower version? Thanks in advance for the help.

-Joel