different ram at the same time

since you said stock is 2400, I guess you have one of these:

Dell Inspiron 3268, 3668, 5675, 5676?

In which pc are you testing the new 3200 ram and getting 2133?

you can try mixing it w the stock 2400 but no guarantee it would run.

What is the exact model number of your system as 24GB of RAM may not be compatible. Keep in mind that if it does work all the RAM will run at the slowest speed which you have mentioned is 2133. Also only 16GB of the RAM will run as dual channel. The lone 8GB will run as single channel. Though all of this is hypothetical since without knowing the model number 24GB may not work.

not surprised. Bios does not like mixing of ram of different spec.  It gets confused trying to find a best compromise. Finally it decides to downclock everybody to a speed slower than the slowest stick in the group.

i have a dell inspiron 5680, the stock ram says 8gb 2400mhz, when i boot to bios it says my ram speed is 2133mhz even though its rated at 3200mhz 

I havent tried to run them together yet, ive just been running the 2 8gb 3200mhz but for some reason bios says theyre running at 2133mhz so i thought it would be ok to add the 2400mhz since it would also be able to run at 2133mhz

i have a dell inspiron 5680, i5 8400 and gtx 1060 6gb, the stock ram is 1 stick of 2400mhz 8gb, ive upgraded to 2 sticks of 3200mhz 8gb but in bios it says the ram speed is 2133mhz

Someone else had this experience before.  It all depends on the ram detailed spec, not just advertised speed.  https://www.dell.com/community/Inspiron-Desktops/Inspiron-5680-retail-ram-speed-slower/td-p/7533895