You need to find the lists on a site. Have you just been guessing lol!

No I haven't been just guessing. I tried some lists I have in the guide and from what I found on the internet. Although when nothing worked I did start guessing.lol
Do you have to be at a certain distance and do you have to do the combos really fast or something?

Whenever you do find a list of fatalites, be sure when you start (when it comes to MK1-MK4) to hold block while you do them. It makes things a bit easier in terms of not jumping and stuff like that for fatalites that require you to press up for example. MK: DA and MK: Deception fatalites are so easy to preform.MK3 combos are very easy to preform. They are just dail up as with MK: DA and Deceptions. MK: DA and Deception's combos are a bit tougher to preform because they require memory to learn rather than MK3's 3-8 hits per combo. If you are new to the MK world Mk3 combos is a good MK to get used for combos. Also once you get good try UMK3 and MKT since there are more characters. Yes for all MK games , but MK: DA are you require to stand at a certain distance to preform a fatality. Keep in mind that it is a good idea to learn one character at a time, not just try a character beat someone and look in a book or something when it fatality time. Focus on one character and before you know it you will know all fatalites in each MK game.

Most fatality lists will tell you the distance you need to be. Basicly there are 3 distances close(right next to the opponent) sweep (roughly a jump backwards away from the opponent) and far which is usually about half screen or further. A few fatalitys have no range and you can do them anywhere but most fall into one of those three.
I assume you are waiting to do these till it says finish them. Other than that the only thing I could see you having an issue with is speed of input. You may just be doing them too slow.

i believe its every MK besides MKDA that yes u HAVE to be at a certain distance. in MKDA u DO NOT go into fatality stance otherwise it wont work. for MKD u dont have to go into fatality stance but it really helps. but u have to be at a certain distance. wither close, sweep, or far. close is right up next to them. sweep distance is the distance between u win a round starts. far is the whole screen. its easy to go into what ever distance u need to, go into fatality stance then, and enter the combo fast. u dont really got much time to do it either. all fatality list for MKD should show what distance u gotta be at for each fatality like say "(sweep)" or the other 2. hope that helps ;)

Thanx a lot everybody. These suggestions you guys posted gives me something to work with now, a direction.
My two sons are counting on me to do fatalities so I need to step up. Mind you the two boys are only 4 and 5 so I'm not sure their even supposed to be playing this game but they are hooked I tell ya! Now their sending me off on a mission to obtain all the MKs that ever existed. I feel like a Shujinko.
Off I go to 'try' and do some fatalities...thanx again.

1: defeat your opponent
2: wait for Finish Him/Her to appear
3: be at the correct distance of the fatality indicated by the list you are using
4: Do the button combination quickly but not so fast that you screw it up...sometimes you gotta tap it out carefully so everythign registers
5: enjoy the slaughter