the trans has some small metal balls that change the direction of the oil to make your trans shift and stuff
so.... what is the problem?
with the pass of the time, and dont change the oil regularly, the oil loose detergent and other chemicals and it starts to make sludge, that make the metal balls cant move freely and the oil cant pass properly to make the correct funtion
if you change the oil you can fix your problem but beware you can also damage you trans turn it to death, new oil came with a lot of deterget and it will wash your trans but all those dirt can stuck even more your trans and maybe need rebuild
consult a professional trans shop for better details
