Outbound route trunk failure

I have looked at this at least a dozen times.

Rhino RCB4FXO/1

2 analog lines
1 - 4channel SIP trunk

Trunk Sequence for Matched Routes:
0 Dahdi 1 (analog line 1)
1 SIP1 (SIP trunk)
2 SIP-BU (Backup SIP Trunk)
3 Dahdi 2 (analog line 2)

About 50% of the time if both analog lines are in use (busy), the outbound call will just bypass the SIP trunks, and ring busy.

I have setup another outbound route with a prefix where the trunks are set to the SIP trunks, if you dial that after you get the outbound busy, the call goes through fine.

Why is the trunk seq. skipping the SIP trunks, sometimes?