Check with your memory supplier about compatibility, or replace the RAM with memory tested to work with your system.

https://www.crucial.com/compatible-upgrade-for/dell/latitude-e7440

You're not being very specific about the RAM you've purchased.  Was it selected specifically for your system using their memory configurator, or not?

There are a number of possibilities:  compatibility, speed or density mis-match, or even a marginal memory socket that works with the smaller capacity module but not the larger one.

The question is way the original memory 4GB doesn't work with the new 8GB from Crucial.

That's not the specific I was looking for.  What is the Crucial part number of the module you purchased?

It's just about impossible to buy on "spec".  That's why memory manufacturers such as Crucial test and certify model-specific RAM.

Bottom line:  if you were to purchase another 8G module that matches the new one, it's likely to work.  Since you know the new module is not compatible with the original, if you have return privileges with your memory supplier, a better course of action is to purchase a new module or pair of modules that's tested and certified to work in your system.  

This system was released at the edge of the transition between DDR3 and DDR3L memory -- also be sure you're not mixing an original DDR3 with a new DDR3L (or the reverse).

Let me be more specific.
my laptop comes with 2X4GB
I have bought a 1X8GB compatible with the Laptop spec.
when I placed the new one I have bought all works fine.
But it is not working if I am adding to the new 8GB,  additional 4GB from the original set I have, trying to have a total of 12GB.

this is what I bought

Welcome to the Dell Community @tzvic 

12GB is a valid configuration.

4GB

8GB

12GB total

The 4GB is High Density (4 chips per side)

The 8GB is Low density (8 chips per side)

Both are PC3L-12800s

Best regards,

U2

See above.  What is the type of RAM you originally had (is it DDR3L, 1.35V or DDR3, 1.5 V)?

 this is the original 4Gb

Here are the details on this model soDIMM.

https://memory.net/product/hmt451s6bfr8a-pb-sk-hynix-1x-4gb-ddr3-1600-sodimm-pc3l-12800s-single-rank-x8-module/

Look up the details for the new one (you've provided information too vague to do so) and check to make sure the timing specifications match.  If the new module has more aggressive RAM timing, it would explain the failure of the two modules to work together.