I started with Lightroom and the mobile App with one month free for testing. Sync went well and I could see all pictures on mobile. After the month was over, I signed up for lightroom BUT with another email. I use the same old catalog on my Macbook Pro (End 2011 - 10.11.06) but logged in with the new email account. Now the pictures that were synced before to the email that I used for the test aren't syncing to the new mobile account.
If you have all the images saved in a catalog, then you can sign-in using the new email on your desktop for mobile sync, then sign out from the Lightroom mobile from the phone and sign-in using the new email address. This will re-sync your images from desktop to mobile using the new email address.
Also, you can merge two Lightroom catalog into one and then you will have all the images in one catalog.