Read this carefully:
https://techdocs.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-monitors-reference-12-0-0/2.html
Check time until up, manual resume, ramp up time, etc. Also, be sure to check the timeout and monitor check settings.
Finally, you are making a very very BAD assumption. Ping is not a good monitor. If a web server reboots, and even the BIG-IP has a web server, the device will respond to Ping way before it responds to HTTP. Why? Device is rebooting and the HTTP service has not yet started!!! So even if you have manually marked the server as up immediately via Ping, it doesn't mean the backend server is ready to accept HTTP connection and indeed the client may be experiencing an outage at that point.
Nothing in life is quick, have patience 🙂