Take a look at the original sentence again:
Richard squeezed, thumped, and shook the empty shampoo bottle, hoping for
a few drops to wash his dirty hair, since
the bottle was completely empty, Richard worked up a lather with a few sprinkles
of bathroom cleanser instead.
You wanted to fix it this way:
Richard squeezed, thumped, and shook the empty shampoo bottle, hoping for
a few drops to wash his dirty hair, consequently,
since the bottle was completely empty, Richard worked up a lather
with a few sprinkles of bathroom cleanser instead.
A comma splice contains two complete sentences incorrectly joined with a comma. Richard squeezed, thumped, and shook the empty shampoo bottle ... begins the first sentence. Since the bottle was completely empty, Richard worked up a lather ... starts the second sentence. Just because you add the transition consequently between these two parts doesn't alter the fact that you still have two complete sentences joined with just a comma.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.