Also make sure that NTP traffic isn't blocked by any firewall. Below some sample commands and output to troubleshoot your NTP problem.
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config tmsh show /sys service ntpd
ntpd (pid 27976) is running...
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config tmsh stop /sys service ntpd
Shutting down ntpd: [ OK ]
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config tcpdump -nni 0.0:nnn host 192.168.1.5 and port 123 &
[1] 30708
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on 0.0:nnn, link-type EN10MB (Ethernet), capture size 96 bytes
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config ntpdate -q 192.168.1.5
10:25:22.453506 IP 10.10.10.10.33999 > 192.168.1.5.123: NTPv4, Client, length 48
10:25:22.468377 IP 192.168.1.5.123 > 10.10.10.10.33999: NTPv4, Server, length 48
10:25:22.469014 IP 10.10.10.10.33999 > 192.168.1.5.123: NTPv4, Client, length 48
10:25:23.453751 IP 10.10.10.10.33999 > 192.168.1.5.123: NTPv4, Client, length 48
10:25:23.468290 IP 192.168.1.5.123 > 10.10.10.10.33999: NTPv4, Server, length 48
10:25:24.453573 IP 10.10.10.10.33999 > 192.168.1.5.123: NTPv4, Client, length 48
server 192.168.1.5, stratum 1, offset 0.000086, delay 0.04105
16 May 10:25:25 ntpdate[30728]: adjust time server 192.168.1.5 offset 0.000086 sec
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config jobs
[1]+ Running tcpdump -nni 0.0:nnn host 192.168.1.5 and port 123 &
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config kill %1
6 packets captured
6 packets received by filter
0 packets dropped by kernel
[1]+ Done tcpdump -nni 0.0:nnn host 192.168.1.5 and port 123
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config tmsh start /sys service ntpd
Starting ntpd: [ OK ]
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config tmsh show /sys service ntpd
ntpd (pid 30809) is running...
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config ntpq -np
remote refid st t when poll reach delay offset jitter
==============================================================================
*192.168.1.5 .PPS. 1 u 3 64 1 14.151 -0.303 0.298
[root@nielsvs-bigip:ModuleNotLicensed:Active:Standalone] config