Why can't my dog jump down as easily as jumping up?

I am going to agree with Maxi.  My vet TOLD ME that jumping DOWN puts ALL the body's weight on just the 2 front legs pasterns and shoulders - and it often more uncomfortable for a dog than jumping UP is.  (Do you like landing flat-footed on a hard surface?)  The THIGHS (in both people & dogs) are some of (if not the strongest) body muscles.  It is often easier to do a "take off" than the landing for most dogs.  Think about how many Olympic sports are done like long jumps, hurdles pole vaults, etc that require a strong push-off or take off utilizing the thighs.  Even in floor gymnastics, there are many body rolls, swings, pivots but only a very FEW runs where they flip and "hope" to land ON THEIR feet.

I do agree with the other poster =>that if she USED to be able to jump up and down EQUALLY, but is (NOW) evidencing a CHANGE (in behavior) well she may need a trip to the vet to check for bursitis, arthritis, or eye sight issues.

That is a sure way to cause broken legs.  Jumping down off the couch, bed, has broken more dog legs than anything else.  Make a ramp or steps for the dog to go up & down on to avoid broken legs.  A ramp for the bed, a ramp for the couch.  This is too dangerous to keep doing.

She might have joint pain.  So when she jumps down the impact with the floor causes sharp pain through her legs.  Next vet visit ask them to check for arthritis.

Using different muscles... back legs to jump up with no pressure landing on a soft furnishing, front legs to jump down landing on a hard surface, so causing landing pressure/pain..either had or has a front leg muscle injury or something worse and/or is jumping onto a slippy surface which the dog knows can cause injury