why is the right channel on my cassette deck inaudible?

why is the right channel on my cassette deck inaudible? ive tried new cables and other cassettes.

sometimes, though, you play a tape, and you can hear the right channel. then, you stop it, and play it again, and you now cant hear it.
Chances are better than even that a connection became iffy inside your probably now prehistoric cassette deck. If you're not good with a soldering gun, track down somebody that does electronic repair in town.
@joenies thanks, ill take a look at it

@edcyn - i already checked the connections; everything is fine there
@joenies how would you know if it is out of alignment, and how would you fix it?  

im sorry, im kind of new to cassette decks
you heads are DIRTY!

q-tips, alcohol, be thorough!

while your at it, do your ears, q-tips warm water not alcohol, check the right side!!!
Yea you need to keep it clean, probably fix you right up. Q tips etc. Mr elliott has the right approach.. First things first..

No need to overhaul it when it's a (GULP), oil change, ay?

Could be the heads just need a good cleaning.   Tape players all need that periodically. Lots of oxide accumulates on heads from tape over time.  Always. 
On my cassette deck Nak 680 ZX I can bias my heads for proper alignment. You might need to have it serviced if your deck does not have head adjustments.
A lot of good info has been given, check connection and clean the deck.
Be careful using q-tips some use an adhesive to attach the cotton. I get my q-tips from my local electronic store, they are made for cleaning electronics, no glue. I. Also use denatured alcohol as a cleaner. I was informed years ago that regular rubbing alcohol can leave a film on the surface it is cleaning.
Other things it could be: Volume gain adjustment is faulty or input source is faulty.
Once you have cleaned and checked all your connections and problem continues send it off to get it analyzed.

Don't use a "soldering gun", those are bad news. If you're going to do your own repairs, invest in a good pencil iron.
tried to re-align head with screws... very tight. couldn’t screw them left or right to save my life.

cleaned heads with alcohol & qtips, did nothing