Recently I bought 12 hole Hohner chromatic. First

First and foremost i recommend you take your harmonica to a professional for repair. But the problem can be many things that you can fix with a little patience. FIrst youll need to open up your harmonica and look at the condition of the reeds(the plate with the slits.) If they are rusted or warped they have been poorly maintained as water will rust the metal(saliva.) Sometimes simple sanding solves this problem though it seems thats not your problem. Usually the reed is misaligned due to excessive use and a professional would fix this with an index card sliding it very slowly under each tip of the reed attempting to level out the small piece of metal. Your attempting to realign the reed with the entire board. To see if your fix has worked you can blow into the reed and see if a smooth vibration comes out. Any more complex solutions would honestly show that its time for either a new reed plate or a new harmonica. For future reference make sure to get all saliva out of your harmonica when done playing it and keeping it in its case. Good luck

Look on YouTube for "harmonica cleaning".

There are good videos explaining how to clean harmonicas on YouTube.

Search for "harmonica cleaning"

Sounds like you need to flush teh unit out as it has contamination. You might try using mild detergent and DE-IONIZED water.
I would first search google for "cleaning harmonica" for suggestions. You need to clean the reeds and NOT leave any residue. DE_IONIZED water should not promote rust or corrosion if dried quickly after cleaning and rinsing. I would lube the slides with CRC226 available in the electrical dept of Home Depot.

There probably is contamination on that reed.

Use google here for access tp c;eaning info:

http://www.google.com/search?hl=en&source=hp&q=cleaning+harmonica&aq=0&aqi=g1&aql=&oq=cleaning+harm&gs_rfai=Cq4YnhKo8TM3pGIGgjgPstOyMDgAAAKoEBU_QYhWt

Probably just needs cleaning. Search google for cleaning instructions. Dirt or deposits on the reeds increases the mass and will cause them to go flat in pitch.

You might find the info you need here:http://www.harmonicalessons.com/repairs_chart.html It sounds to me as if you have some stuck reeds.

You can take it apart by removing the two screws, see if any of the reels seem blocked or obstructed by dirt, bits of dust, oxydation (greenish crystalline stuff) or similar, try to remove the stuff that seems to obstruct the reel with a medium hardness paintbrush (you might want to use some alcohol), but be very careful not to permanently bend or otherwise damage the reels, it will ruin the sound of the harmonica.

regards

Triarcuate

Dis-assemble and inspect for contamination.

clean it..the reeds are probably sticking closed from spit