These days, pretty much everyone uses emoji to express themselves. There are now many different emoticons and you can download many more yourself. Not every emoji is always properly displayed to your interlocutor. What causes this and what can you do about it?
You have undoubtedly experienced it, someone sends you an email with something that clearly should be some kind of smiley or emoji, but all you see is a strange symbol or a letter, for example the letter J). How come some emoji are not shown? The simple answer: because they aren't emoji.
That's why emojis aren't shown
We are so used to the fact that we can all send a standard set of emoticons to each other that we no longer realize that there is nothing obvious about it. A few years ago, that didn't make much sense. When someone sent you an emoticon (that is, before the introduction of the emoji) it was more likely that you couldn't see it than you could. The reason for this is that the condition for displaying such an emoticon is that you have that same emoticon installed on your system.
The joke is that people usually don't send you an image at all, but simply a command that activates the display of that emoticon on your system (this saves data traffic, because the image does not have to be sent). That is the power of emoji and the standard integration within, for example, iOS and other platforms. Everyone has exactly the same emoticons installed, which is why the emoji someone else sends is exactly the emoji you see.
So with this in mind, it makes sense that when someone sends you an emoticon that he/she has installed on his/her system with software, it will not show up on your system (because you don't have that software). In short, don't worry: it's not you, it's them!
If you can't get enough of emoji and if you want more options within WhatsApp to express yourself, you can also use stickers instead of emoji. These are actually larger emoji and are therefore displayed differently. A sticker is a picture and therefore not standard 'baked in' in the software. Read more about WhatsApp stickers here.