castelnuovo
...what is the best way to figure out why?
Sometimes it is easy and sometimes I trim and trim and trim and it still doesn't fit. Then I get crancky and then my wife tells me that I am getting crancky which makes me feel more crancky and the darn piece still doesn't fit.
This scenario (among other things) is what separates the kit builder from the kit assembler... Approaching it from several different trouble-shooting angles is a skill that has to be developed.. Simply avoiding it will not help you advance in skill-level, although it's gonna keep you from re-enacting scenes form Platoon with Charlie Sheen...
It always depends on the part though.... Might be something as simple as a bit of sprue hitting something, or even that you're trying to to fit the wrong part into the wing.. (Don't laugh.. EVERYONE has done it, lol) Check this FIRST, before you go cutting, hacking, sanding, etc..
Anyway, I'm pressed for time... Will expand further upon my return from the Doc..
EDIT:
Ok, back at keyboard... What doesn't fit? Is it something that's going to be unseen after assembly? If so, then don't sweat it.. If it's something that another part depends on, then you got a problem.. You're gonna have to, as Reasoned put it below, get a persuader...
Trimming and/or sanding-filing should take care of 99% of your fit-issues though.. But if it's something like the Monogram P-61 canopy, then another approach id required, and that's using strip styene to "raise" the fuselage up to the canopy rail. You can also try filling the opening, but if you don't get the putty right duirng application, you'll have to sand in an area that will casue you to possibly damage to the surrounding detail...
Another thing that happens, especially with older kits and their "operating features", like movable control surfaces (found on a LOT of Revell's 1960s-era 1/32 kits)... Many times the control surfaces don't want to mate-up with each other or they have a big, out of scale gap in them, between the wing and the aileron (this includes the elevators and rudders too)... You'll need to lengthen the control surface with strip styrene, or add on the wing-area surrounding it, using putty.. If the control-surface is too wide, span-wise, then it's just a matter of filing/sanding it to fit.. Same goes for chord-wise..
Parts that don't mate-up well because of the locating pins means, cut off the pins... Use a slower-setting cement for those, like Testor's Black Bottle, and just cement the parts a little at a time, clamping tight as you go around that area until you get it complete...
There're a LOT more things to do, and solutions for them that will only come by doing, and only building "Shake & Bakes" won't teach you anything... Can't tell you how many people I've read who get stymied by little, easy-to-solve issues, issues that they wouldn't give a second thought about had they only built a few dozen Lindberg, Hawk, Aurora, Revell, Monogram, and other so-called, "inferior" kits first... And God help them with a limited-run kit too...
Bottom line is that you won't ever become a "Master Modeler" by building only Shake& Bakes... You actually NEED to build some kits the you have to beat into submission, lol....
So.. Up for a Modelcraft 1/48 F-82? THAT kit you'll wanna have the wife & kids outta the house for... You HAVE to be at least an "advanced" modeler just to get the parts offa the sprues... If all you want to do is throw a tube of glue in the box and shake it, stay away from this one... But man.. No way, I HAD to have THIS Pony in the stable... Boy, is she ever a beauty once the swearing is over...