I think your doing good to only have the 20 minutes then she's asleep!

Charlie went thru this stage - i used to put him down and then i would look and he would be asleep. He was an angel! Then i reckon at about 5 months we went to darwin and i didnt' want him to cry and wake the rest of the house up so i got into the habit of rocking him - i regret that - he is a big boy and weighed 11 kilo's at 5 months - so wasn't good for my back. In the end i would put him in his bed and pat his back for a little while and walk away, if he started crying i would come back in and pick him up, cuddle him and then put him straight back in his bed - I had to do this for about a week straight - in the end he just stopped crying because he knew it wouldn't get him out of going to sleep. Worked for me, I didn't like letting him cry.

Good luck!

P.S Your little girl is gorgeus