I know it's a little late, but I found a solution to the Hilton to TripIt sync issue! Although Hilton no longer emails all the time (especially when it's a third-party booking) and no longer offers an email option on the website, you can still send a reservation email from the app!

Go to Stays in the Hilton iOS App, tap the share (arrow in box) icon, and then select Email. Sent it to [email protected] and it will sync properly. I imagine there's a similar function on Android.

Hope this helps!