Once in a while the F5 will stop responding to iControl REST api calls. Restarting some of the services on the device will help:
tmsh restart /sys service tomcat
tmsh restart /sys service restjavad
tmsh show /sys service tomcat
tmsh show /sys service restjavad
F5 utilizes iControl REST API as part of their automation toolkit. REST API is a powerful way to automate F5 management. iControl REST API was introduced by F5 in 11.5 code version. 11.6 code version is the first major code version with a relatively stable release. However, 11.6 does not support remote authentication like TACACS+. For iControl REST API with remove authentication, it is important to utilize 12.x code version. F5 programmability training documentation and related information are available here.