Sounds like you're doing nothing wrong but like when i first started making my own CDs, you haven't realised when the disc is first made and you finish recording it, that's it - the disc is "finalised" by the recording software which means there is no room left for anything else and that it's not recordable on. Takes a bit of getting your head round if like me, you're used to audio cassettes where you record half of one side, decide which other tracks to add, then record those and then start on the other side! The only way i found is to put together the disc contents over a period of time then burn it when it was full/near full.