You should not have to kick down to descend you should be able to descend with proper weighting, do not continue with this method as it is not the proper way to descend.
1. Perform the weight check. Deflate your BCD while inhaling holding that breath, and you should float about eye level. If not add/ remove weight until you are eye level. This is not for every diver as I have had some divers that were to heavy when at eye level. As mentioned this is not exact but a good place to start
2. Ensure you actually have let all the air out of your BCD, try your dump valves as well.
3. What type of exposure suit are you wearing? if it is a wetsuit then is it the same as the one you used in your training? You will find with different wet suits that you will need to adjust your weighting, thickness, full length vs shorties, age as they will become compressed and loose buoyancy over time, and also the manufactures or custom shops as the quality of the neoprene may vary.
4. Your breathing may be affecting this as well, I have had divers that would exhale extremely slowly while descending and would keep them a bit buoyant until they fully exhaled.
5. If you are at your safety stop and start to ascend and you are not fining or exhaling extremely slowly causing you to become buoyant and ascend you will need to add some weight. Ask you buddy to watch you you may not realize you are fining or exhaling extremely slowly while you try to focus on your buoyancy.
With this case I would start with 1 - 2 kg, normally I have found 1kg to solve the issue but have had some need 2kg, if you start with 2 kg put one kg in your BCD pocket and tell your buddy before you enter the water you may ask him to hold that 1kg while you check your buoyancy during your safety stop and determine your weighting.
I am not suggesting over weighting as this can lead to safety problems, inability to stay buoyant at the surface creating a possible panic diver, harder to swim at the surface, buoyancy problems under water, and rapid descents; so if you are not sure you should speak with your instructor so they can evaluate you further in water or he can talk to you about any issues he may have seen during your course, or consider taking a peak performance buoyancy course.