When you sync the first time, you have to make a decision whether to sync from website to app or from app to website. Maybe you chose the wrong option? If you have a freshly installed app with no info and then sync TO the website, it'll delete everything.
I have installed FS on my phone several times, and synced with no trouble. I do, however, make absolutely sure to pick the right option.
"Losing weight is never about eating as little as possible"
"Eating enough, moving more, not eating stupid s**t = losing weight."
"Do. Or do not. There is no trying."
- Master Yoda.
I started my weight loss journey on October 4th, 2010 - and this is what I have accomplished so far: