Modelling NCP1217 IC for LTSpice
The first thing to do with any spice simulation problem is check your netlist, which is the easiest way to debug with graphical spice packages. If you don't have a .lib and the name of your downloaded package then you have a problem. (you can do this in view->spice netlist)

The second thing is you need to get your paths straight. If you do have a .lib statement, then the path of your .subckt file needs to follow the .lib statement. Example: if your .subckt file was named NCP1217.cir (or whatever its extension is, they are all text files anyway) was located in C:\LTC\libraries then the spice command that needs to show in the spice netlist is:

.lib c:\LTC\libraries\NCP1217.cir


If you put your file in the LTspice\lib\sub folder you don't need to provide the entire path.

There are ways through the attributes list (in your symbol files, .asy to be more specific) to include the .lib statement. 

The last thing is you need your X line in the netlist to have the appropriate nodes, double check each node of the lt spice netlist and make sure it matches up with the netlist in your 3rd party model. Then if you have multiple devices, the last thing should be the name of the subcircuit.

Here is an example:
     XU1 posrail negrail posterminal negterminal output DEV1376

Keep in mind that no spice model will even come close to the real world, most all models are simplified in spice to model selected parameters in the datasheet by the manufacturer and only approximate the real world. (after all if they really did a transistor model of each device, all you would have to do to create a copy of the device is download the spice file) Some models will give some kind of disclaimer as to what the model can and can't do.