I’ve delegated X amount of DEC and now after undelegating, the DEC hasn’t been returned to my wallet.
There is a 21 day waiting period after undelegating DEC. This doesn't apply if you redelegate. Any time you wish to undelegate your DEC from a validator and back to your wallet, you'll need to wait 21 days for it to return to your wallet.
You can read more about staking in our medium article: https://decentrnet.medium.com/delegated-staking-via-the-decentr-browser-is-now-live-d0973fd94c47