What is it doing if you just give it a regular push? Does it go to close and just bounce back open or does it latch...but not "shut" latch? Hard to explain what I mean. You know how you can close the door where it won't open, but it isn't shut all the way. Is it doing that? If it is then perhaps the locking part on the door is slightly loose and over time has started moving out of whack so it doesn't close poperly. Give the WD40 a try, maybe it is gunking up and that will eat it away. If it does, shoot it with some silicone afterwards or you will corode right up. If it latches but doesn't shut all the way you are out of alignment, just need to fiddle with the latch on the door or the post on the car, make sure both are tight.
On a side note, I am not sure what your post looks like (the part that is on the car side of the door latch) but some (maybe all) have a plastic sleeve on them. I have had those just wear out and fall off, and when that happens you have to replace the post or else the door doesn't shut well. Good news is it is easy and cheap
