There are several reasons as to why your downvotes may not be registering.
New Account
New accounts have some restrictions on their abilities to downvote. These restrictions pre-date the release of our proactive vote manipulation system, but some are still in effect.
In time, these restrictions will be lifted.
Vote Manipulation
It could be that your votes are being flagged by our system as manipulation.
We monitor for many types of vote manipulation, and take automated action to prevent it.