The answer has been repeated many times. Don't pause after saying the Siri activation word. Speak in one complete sentence as "Siri what time is it", not "Siri....What time is it". The topic commonly comes up from previous Android users who had to pause for Google to activate before speaking a command. That is not necessary or even recommended when using Siri.
Ironically, I have also seen posts from users who no longer heard the umhum and wanted it back so they knew Siri was listening. I would suggest to send Feedback to Apple for either of these situations. Maybe they need to have a setting to turn it on/off, or even be able to choose other acknowledgement phrases to use. Since there is no specific feedback category for Siri, I would suggest selecting your device type, then fill out the form for the request on using Siri.
Product Feedback - Apple