Strange Problem with Codecs please help


Its a very strange problem I have got, the thing is whenever I download a movie in parts from any website with a 300-400mb size, in MKV or MP4 format, let suppose I download the following movie from a web site:

ok, some days earlier everything was fine, I mean I just download the first part of the movie that contains mkv or mp4 format , and after downloading its only first part I drag it in media player classic and it played fine till now, but now every movie I download in parts with MKV or MP4 wont play at all even I download all parts and joined them in one file and it wont play at all, but other then this whenever I download a movie that is also in MKV or MP4 format but in parts with winrar I mean

this type of download perfectly play good on any player, even Just extracting first part of the movie and it plays fine. I m sorry if I cant clear my point. I dont no what is the problem here, I have download 5 or 6 movies that are parts like [MKV.001, MKV.002] and I cant play all of them on any of my player, each of them asks for codecs or give me error [cant render the file], but all of my other movies even the above type [parts in mkv or mp4] of movies which I download previous month in my system are playing fine and smooth, I have installed k-lite codecs, ffdshow, and realalthernative at the same time, I try every thing on my own, but the problem is still there I cant just delete my downloaded movies. please help me.