Just make sure your phone is not too far from your watch or you have a good WiFi coverage. If not then there is little gap between times that it won’t sync with the phone. For example, I put my phone at the front desk at work and when ever I went to the back there is these few moments that my phone ring but my watch stayed silent. It’s no big deal, but it’s a thing.
As @AppleHaterLover stated, all all iPhones after the iPhone 6(except the SE) had a Taptic Engine instead of the vibration motor. This is the difference in vibration intensity and frequency seen between the phones