Can't add more tracks to my project

[dominicwest]

I have re-opened a project from last week, hoping to record some extra tracks. I can play them and hear them normally "live" but when i play the piece back today's new additions aren't there. I guess I have clicked something unwittingly in this project that stops me adding more to it but can't figure out what. I am arming the track to record a soft synth and then hitting the record button at the top just like I usually do but its not recording anything new. When I go into other projects, I can add new tracks  to them just fine. Any idea what I have done?  

[John Vere]

This can happen if you have auto punch in enabled somewhere along the timeline. 

And for recording midi tracks you arm the TRACK for record and then press the R key on the keyboard. You should see the data being written as you play. 

[dominicwest]

Thanks very much John. Somehow i had clicked on autopunch- don't know how, I haven't used it yet and that's what was stopping me!