I had similar problems with the progress getting stuck about a third of the way. This was also with about 280K of data, or about 2,000 actions. Omni support said that due to the amount of RAM the iPhone has, they don't recommend syncing more than 1,000 actions. I reduced my database to just under 1,100 (couldn't easily prune it further) or 140K, and since then it's been working perfectly.