Many members have problems alerting other members when they post or reply.
The member you are actually replying to will get an alert. But if you want to alert other members you have to "mention" or "callout".
I have created a short (silent) screen recording to demonstrate this. (I've found it very difficult to describe in just words!)
Click on this link and let the screen recording run:
dropbox.com/scl/fi/12irgqay...
Once you have added all the members you wish to alert, just post or reply as usual! So long as the member names are blue, they should work.
Note: This was recorded on a Mac - and I have shown an on-screen keyboard to illustrate the typing. It might look a bit different on another device but the technique is the same.