Memory should *always* be purchased in kit form of pairs in one package together (2x, 4x, 6x, 8x). There are many reasons for this, but the most common necessity is that memory vendors may use different memory modules for their RAM sticks from production cycle to production cycle. So for example Corsair uses both Samsung and Micron memory. Even on the same model memory, they may one month switch to Micron due to a Samsung NAND memory chip shortage or vice-versa.
Now that doesn't mean same memory models that are not sold in kit form won't work together, but it's not guaranteed. Kit form memory are made next to each other on the same production cycle and are tested at the factory to work with each other prior to being packaged. I strongly suspect that's your issue.
PS: when posting questions for help here, always post your computers full system make and model specs (motherboard, CPU, memory, etc.).