can u check for the following log?
show log iap-bootup | i NTP
basically this shows what happened to NTP while IAP bootsup. Here are samples from lab using 4.2.3
1. default ntp.pool.org
IAP5# show log iap-bootup | i NTP
Launching NTP Client on pool.ntp.org
2. setting ntp via option 42 from DHCP server
f0:5c:19:ca:13:1a# show log iap-bootup | i NTP
NTP server 10.70.100.6 received via DHCP option.
Launching NTP Client on 10.70.100.6
another important thing, which has caused issues in my lab is DNS server setting. IAP should be able to resolve pool.ntp.org. At times one forgets to set DNS server on infrastrucutre devices via DHCP.