Time Machine Back Up Full when its supposed to delete oldest Back up?!?!

Hi all,

i sporadically back up my MBP with Time Machine, so i dont use it hour by hour as some people do but week to week i'll plug it in and let it back up/ I thought the idea was that it deletes the oldest stuff when the drive is full and as long as you know that, its fine.

i havent back my machine for a while so i pluggged it in at the weekend. Time Machine said 'Drive Full'. As above, i thought that the idea was that Time Machine would then delete the oldest back ups. Why is it not doing this?

i have changed out my hard drive for a new one. Could it be thinking that this is a different drive altogether and thus wont delete the already created back ups as it thinks this is a new one? i must admit it doesnt look like this is what its asking?

MBP drive is 256GBs and drive is 320GBs. i dont really need more as i'm happy with a current snap shot of the drive. So as long as it can do this each time, and delete the previous, i'm happy. But it doesnt. Any ideas?