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The first thing to do will be to have the fluid flushed checked for level and quality. If the fluid is low or dirty, it will need to be topped up and/or flushed. If the fluid is fine, then there may be an issue with one of the clutch drums or a solenoid internally which would necessitate further diagnosis and repair.
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Yes the fluid level can drop over time without a lot of noticable leakage. And if you have seen some leakage, this could be transmission fluid.
And by "dirty" I mean it would show signs of metallic flakes which would be the internals of the transmission wearing / shearing over time - not actual dirt from the outside.
Definitely you will want to start with having the transmission fluid checked.
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I don't think that is very likely. The internet and bad word of mouth tends to keep shops pretty honest these days.
The fluid is expensive so I would have it checked before flushing out the whole system.