Sometimes it takes 21 seconds.
Seriously, the motor has to warm up to operating temperature before there will be any output from the telltale, because all the water for the telltale goes through the thermostats first. If you can hold your hand on the head, it isn't warm yet.
Did you have it on a hose, or in the lake?
If on the hose, water will come out of the prop long before it comes out the telltale.
If on the lake, a little spray mist out of the exhaust relief ports indicates it's getting water.
Spiders and wasps have a way with tell tales, also.
To service the water pump you have to drop the lower unit. It isn't hard, but is a bit involved. Get a factory maintainance manual and have at it.
Water pump maintainance should be considered an annual event.
hope it helps
John