Here is the reason for your partition problem

First you must understand that to partition a drive is always done on the primary logical drive. Here is a example, originally you have a 100M drive, when you first partition it to become 20M for C's and 80M for D's then further attempt to partition will be done on the c's which is now the 20M drive. That is why you have only 11M left and the bigger chunk is the rest of the 500M+ which is not able to partition further.

Remedy, reformat everything and partition again with smaller D's and bigger C's so that you can further partition the big C's if you choose to.