How are you asking Siri? Can you give examples? if you ask in the wrong way it might think you are asking for something else.
Requests that work for me....
"HeySiri, remind me to take out the trash when I get home"
"HeySiri, remind me to do laundry tomorrow"
"HeySiri, add tomatoes to the grocery list"
If it is not working even then, try just restarting your Watch. Sometimes it can get a little confused and a good reboot clears that up.
BTW Apple sells the Apple Watch, they do not sell an iWatch. If you have an actual iWatch, that is the source of the problem, right there. 😊