First, bear in mind that Apple forbids direct downloads from the Web to their iOS devices (iPad, iPhone, iPod Touch); they only allow downloads to those devices from the Apple App Store. Buyers using those devices will need to perform their download with a regular computer, then they may be able to sync the file from there to their iOS device.
This help page covers some other factors that can cause slow/stalled downloads:
In your E-junkie Transaction Log, if the IP address of the last download attempt shown in the Download Info column matches the Buyer IP shown in the Billing Info column, that's almost certainly the same person; however, a mismatch in IPs does NOT indicate theft, as the buyer simply may be retrying their download from a different location (e.g., placed their order from an iPad but used a regular computer elsewhere to download), or their ISP may assign them a new IP with every Internet connection session.
It's not really possible to determine conclusively when the actual buyer is attempting a download vs. anyone else, nor is it possible to prevent a buyer from copying or sharing their file after they've downloaded it only once, so our link expiration settings are only intended to ensure that if a buyer shares their link with others, it soon becomes useless to everyone.
If you're selling an eBook in PDF format, you may be interested in setting up our PDF Stamping feature to deter file sharing: