Trouble Getting Dual Antenna Lock (VB3iD/VB3iDR Units Only)

The slip module is most likely not enabled.

Communicate with your VB3iD/VB3iDR using VBOX Setup software.
Enter VBOX Setup, then access the GPS tab, and check the ‘enable slip module box’.
After you have pressed this, the box will re-scan, after which the slip module channels will appear selectable as a tab, in the channels list.

Make sure that both of the antenna cables are the same length. Timing is a very important aspect of the Twin antenna lock and identical cable lengths will ensure that signal propagation delays do not cause unreliable dual lock.
Check the physical separation between the antennas (centre point to centre point). Does this match up to what has been configured within the VBOX Manager.
If the separation is correct, is the location suitable? Try driving to a more open space, free from obstructions or other traffic.
Check the antenna connection with the VB3iD/VB3iDR; only small amounts of dirt in the socket can cause a significant reduction in signal strength. Also check the cable at the plug and along its length for any damage. Do not extend the cables in any way when using two antennas.
Move the antennas away from the edge of the roof, and away from any roof bars or additional antennas.
Try removing and re-applying power to the box.
Check that ROLL MODE is not enabled if using a front and rear antenna layout.
If in ROLL MODE, make sure the primary antenna (Port A) is on the left hand side of the roof.