It does sound as though something odd is going on but it IS possible to alter their lifetime want.
Firstly though it is perfectly normal for a Sim to get a new LTW after fulfilling the first one. It doesn't have to be filled though. You don't have to fulfill the first one if you don't want to.
-In order to alter a Sim's existing LTW you will need to activate debug mode by opening the cheat box and typing in 'boolProp testingCheatsEnabled true'.
-Hold down the shift key and click on a Sim. It doesn't matter which. You should see an option to 'Spawn...'. Click on that and then click on 'Sim Modder'. A little figure of a baby should appear next to the Sim you clicked on.
-Now make sure you have the Sim selected that you want to change the LTW of. Click on the baby and there should be a LTW option there. I think its on a submenu branch. Sorry I can't be more specific.
There should be three options like 'Normal Recalc', 'Cycle All Wants' and a third one.
'Normal Recalc' will choose a random LTW normally available for that Sim's aspiration. 'Cycle All Wants' will choose a random LTW from all available LTWs and I can't remember the third.
-Keep doing this until the Sim gets the LTW you want for them. Then remember to turn off debug mode (boolProp testingCheatsEnabled false)
If you have any hacks or mods in your game then I suggest removing them the next time you plan on creating a new adult/teen Sim, or have one age up, and see if they get a different LTW.