I've bought the ram that dell suggested me. It's a crucial 8 gb ddrl 3 sodimm which should be compatible. I tried to plug them one by one old one one seems to work on both ports but other one doesn't

Welcome to the Dell Community  @Cihan_erkn 

The "New" memory, is it DDR3  or  DDR3L ???

Best regards,

U2

Memory should only be installed in pairs of the exact same type and brand.  Not doing this in most cases results in upgrade failures.

Dell published memory specs:

Memory
Slot Two SODIMM slots Type Dual‑channel DDR3L Speed 1600 MHz Configurations supported 4 GB, 8 GB, 12 GB, and 16 GB

@Cihan_erkn 

If the new one does not work when tested alone in both slots then the memory would be defective or incompatible???

Unless you can test the new memory in another laptop then return or exchange.

Regards,

U2

Ddrl 3 1600 mhz sodimm

I've read on forum about resetting the cmos could that help?

A reset is worth a try.