Failover not switching back to WAN1

[coopertown]

Present Scenario:

I was 2 difference ISPs and their routers.  I set up failover on pfSense via (Tier 1 & Tier 2 w/member down).  However, when WAN1 is down it switches to WAN2 but when WAN1 is up again, it doesnt switch back to WAN2.

Question:  is that a glitch/bug?  Is there an alternative?  What can be wrong?