I am also facing the same problem since couple of months.I felt pathetic to find the inconsistent performance of Google Assistant.
On most occasions i am not getting even the ding tone when i call OK Google, occasionally it will work surprisingly.
More importantly, i have enabled Smart Unlock using Bluetooth, when i ask Google Assistant to call someone, It just vibrates the phone once for most occasions, and would work will on 30%.
What is the problem for Google Assistant to execute the commands on a Smart Unlocked phone ?
With the factory version, it even had the courtesy to notify me that i have to unlock the phone for it to execute my instructions, but with the recent updates, vibrating the phone is all it does to convey the same.
I rely on Google Assistant so much for making calls, this inconsistent behavior made me to loss the confidence on using Google Products.
I am now calling the Samsung Bixby first and once the phone got unlocked, then i would call Google Assistant and give the instructions.
I have to call one assistant to instruct the other to listen and do what i said.
As one assistant won't understand my instructions and the other won't listen and execute my instructions,
I am rocking a Samsung Galaxy S10e.