The first thing to check is if your Apple Watch’s battery health is below average. On your Apple Watch, go to Settings > Battery > Battery Health and see if it’s below 80%. If it is, you should consider getting a battery service from Apple. On the same page, be sure to check if Optimized Battery Charging is enabled. It’s enabled by default, so there’s nothing to worry about here for most people. However, if it’s disabled, consider enabling it to reduce battery aging.
Another thing to check is if your watch’s battery is dipping suddenly. On the watch, go to Settings > Battery and check the graph to see if there are any sudden dips, which you can attempt to correlate with what you were using the device for at that time, so you can determine if there’s something you need to be doing differently.