Hard work is "washed away" - Correct word?

How about nullified? There are two definitions of nullify in most of the online dictionaries and most of them are similar to this :

1 make legally null and void; invalidate.

2 make of no use or value; cancel out.

It seems to me that the second definition carries the meaning you are looking for.

See the Oxford online entry http://www.oxforddictionaries.com/definition/english/nullify

"My hard work always goes unnoticed..."

Unnoticed — Cambridge

adjective, adverb Without being seen or noticed
"His contribution did not go unnoticed."

Escape notice — TFD

Fig. to go unnoticed; not to have been noticed. (Usually a way to point out that someone has failed to see or respond to something.)
"I suppose my earlier request escaped your notice, so I'm writing again."
"I'm sorry. Your letter escaped my notice."

How about erased, obliterated, destroyed, or ruined?

"My hard work is always [erased, obliterated, destroyed, or ruined] by client's repetitive changes. Other employees' hard work is visible to you, but not mine. So everyone here got increment in the last 7 months but not me."

From Dictionary.com:

erase: to obliterate

obliterate: to remove or destroy all traces of; do away with; destroy completely.

destroy: to reduce (an object) to useless fragments, a useless form, or remains, to render ineffective or useless; nullify; neutralize; invalidate

ruin: to injure (a thing) irretrievably

As to the lack of an "increment" compared to other employees, perhaps their work is not washed away, erased, obliterated, destroyed, or ruined by client's repetitive changes to the same extent as is the OP's.

Consider obscured (the verb form)

1 Keep from being seen; conceal: gray clouds obscure the sun

1.1 Make unclear and difficult to understand: the debate has become obscured by conflicting ideological perspectives

1.2 Overshadow: none of this should obscure the skill, experience, and perseverance of the workers

Oxford Dictionaries Online