VCR not working for some cassettes
What do you mean by "not able to play my VCR with some cassettes. " Is it that some tapes do not display images? No Video? From your name it seems that you are from India, Assuming that to be so, I also presume that you must be possesing a Multi-System VCR, to play back PAL Tapes from India, and NTSC tapes from the US.
And if that is the case, I guess it is a simple problem of resetting the System standards. Please go to the Main menu of your VCR using the Remote, and select AUTO in System (You will be given choices of AUTO, PAL, NTSC 3.58, NTSC 4.43 and NTSC PB PAL.
Likewise, it is entirely possible that your Television SYSTEm Standard has been accidentally changed. If it is Multi System TV,
Then, again, using the Remote of the TV, select Menu, and then select AUTO in System.
In all likelihood your problem should get solved.