There are a few main reasons you might see buffering:
1) Your computer can't process the movie fast enough
Most modern computers can process movies at any of the quality settings significantly faster than you watch them. However if your computer is busy doing other stuff, or just a bit old then it might struggle. If you stop watching the movie, you can see how much has been processed in the movie list page. Try waiting until 10mins has processed, then tap to watch again. If you're buffer-free for at least 10 mins, then this shows that the issue was processing speed.
Your options are to either set movies processing in advance, or to try watching at a lower quality level (which is quicker to convert)
2) Your network connection can't transfer the movie fast enough
Most wifi connections are pretty fast - but if you live in an area (like a block of flats) with lots of wifi routers, or you are far from your router then it might struggle to transmit the movie as quickly as it is needed. Similarly, if your network is working hard doing other stuff (like running p2p connections), then it may be overwhelmed.
PC Pro has a good article on improving your wifi connection
Your options here are to either work to improve your wifi, or to try a lower quality setting (which requires less data transmission).
3) You are using an older iPhone or iPod touch with high quality conversion settings
If you are using really high conversion settings (particularly with 720p and above for the size) then the older iPod touches and iPhones might struggle to decode the video fast enough.
To give some examples here, these are the devices I test on
Your options if you are getting buffering here are to reduce the resolution or quality setting. Try from the lowest setting and work up.