another troubleshooting test to try; rename your 'world' folder and generate a new one (to see if its something in the world like a chunk error or rogue block or redstone mech).
- you are using non-standard port 25575. are you definitely trying to connect to that? i.e. play.whatever.com:25575 - are you home hosting? and portforwarded so your router sends traffic for 25575 to the machine hosting the server?
can you build anywhere in the world? if yes, then maybe another plugin is protecting a bigger spawn area.
if not, go outside of spawn where you want players to be able to build and do /region info it will show if there's any other region (& so other rules) preventing building.