APM OTP not being received

We're investigating an issue where OTP isn't being recieved by users. The logging just seems to to suggest a black hole. User confirms not in junk etc. This doesn't happen all the time, it is quite sporadic.

Apr  6 10:46:19 BIGIP1 notice apmd[14867]: 01490010:5: /Common/Citrix_XenDesktop:Common:82caf589: Username '*removed*'
Apr  6 10:46:39 BIGIP1 notice apmd[14867]: 01490115:5: /Common/Citrix_XenDesktop:Common:82caf589: Following rule 'Pass' from item 'Firewall' to terminalout 'Pass'
Apr  6 10:52:54 BIGIP1 notice tmm2[22524]: 01490502:5: /Common/Citrix_XenDesktop:Common:82caf589: Session deleted due to user inactivity.

A successful one reads as:

Apr 7 05:03:48 BIGIP1 notice apmd[14867]: 01490010:5: /Common/Citrix_XenDesktop:Common:35ba7aa7: Username '*removed*'
Apr 7 05:04:21 BIGIP1 notice apmd[14867]: 01490115:5: /Common/Citrix_XenDesktop:Common:35ba7aa7: Following rule 'Pass' from item 'Firewall' to terminalout 'Pass'
Apr 7 05:04:59 BIGIP1 notice apmd[14867]: 01490115:5: /Common/Citrix_XenDesktop:Common:35ba7aa7: Following rule 'Successful' from item 'OTP Verify' to terminalout 'Success'
Apr 7 05:04:59 BIGIP1 notice apmd[14867]: 01490220:5: /Common/Citrix_XenDesktop:Common:35ba7aa7: Pool '/Common/Pool_A' assigned
Apr 7 05:04:59 BIGIP1 notice apmd[14867]: 01490005:5: /Common/Citrix_XenDesktop:Common:35ba7aa7: Following rule 'fallback' from item 'Pool Assign ALGCTXA' to ending 'Allow'

Is there any additional logging that can be put on to see what is going on with sending the OTP email?

Thanks in advance