Does it lock on and fall off easily?

Mine locked in solid and stayed in place until removed. This is after the original one was coming off randomly when in operation. I’m assuming due to age and wear in that area where it locks together.

How do you remove these from pool sweep. I have an incredibly difficult time removing it without having to rock it too hard to remove it

These are not hard to take off of the Polaris cleaner. At times, it will pop off by itself if you don't get it on good.

how can i find the model number of my polaris?>>

Check the connection of your bag. Most Polaris have either square connections like this (for 380/360 Polaris) or a round connection for 280 Polaris. There are newer models which use other types of bags.

I need to return mine as it is three inches across and i need four inches. how and where do i return it? and is it possible to get the four inch bag?

not sure about getting another size but returns are very easy . Just go to your order page ,you will see item , has questions , just click return item and you can print out mailing label . Also many items can be returned to Kohls