Should I be worried if my implant still hasn't completely dropped after 6 months?

At 6 months, the amount of future change expected is minimal for most women. Small, tight breasts do take longer to stretch and accomodate the implant than larger breasts and thus you may see some improvement in the coming months. From the photos, your implants do not look terribly high, but there is a nipple position difference. It is difficult to tell from your photos (especially without a front preop), but it is common for augmentations to reveal asymmetries between the breasts. These often become more noticeable after implants. Even a minor difference in nipple position can look significantly asymmetric after an augmentation. A simple lift on one side can correct this.

I don't think I would expect much change from hereon out. From the before photos it appears that there is some limitation of lower pole of the breast, and with as large as an implant as you have, more than half will be above the nipple line. Many would find this a good result, but if the upper pole fullness disturbs you, you may need to consider smaller implantsAll the best.