Two key members of West Germany's Cabinet are in Washington today on missions aimed at smoothing out relations with the United States that have come under increasingly severe and uncharacteristic strain since the Carter administration took office.

There is more than a touch of bewildderment among top officials here over the range of issues that have rather suddenly come between the two most powerful members of the Atlantic alliance - countries that have rarely quarreled in the past.

Criticism of the new U.S. administration's stance in several fields is also growing in the German press.

(In Washington, after talks with President Carter and Secretary of State Cyrus R. Vance, West German Foreign Minister Hans-Dietrich Genscher strongly denied talk of a crisis in German-American relations. He said his "more than satisfying" meetings did not concentrate on the disputed German nuclear deal with Brazil, but covered a wide range of common concerns. A White House statement said Carter and Genscher agreed on the need for "the continued vitality of the Atlantic alliance.")

Most top officials seem to feel, or at least hope, that the problems are temporary, because both nations are so important to each other.

At the moment, however, there is no solution in sight to the following sources of bitterness:

Carter administration pressure on the Germans to back away from at least part of their $5 billion contract to supply Brazil with atomic power plants and nuclear technology.

Apparent U.S. moves to break an understanding reached by the previous administration that thousands of new battle tanks being built in both countries would use at least some common parts - such as guns and engines - to further the cause of NATO standardization.

German reluctance to make a decision on buying, along with other NATO countries, an expensive U.S. built early warning aircraft. This reluetance is based in part on lingering concern that the highly expensive aircraft fleet will not work well, but it is being greatly strengthened by resentment over the tank problems.

Growing anxiety here over what is regarded in some quarters as "amateurish" handling by the new administration of such as sensitive issue as human right around the world.

The criticism is not directed so much at President Carter and his statements as at things like the recent withdrawal of the U.S. protest concerning Soviet dissidents at the U.N. Human Rights Commission, the unsolicited apology for the alleged U.S. role in the overthrow of the former Chilean government, which also had to be retracted, and several statements by U.N. Ambassador Andrew Young that either heve not been backed-up or have been contradicted.

Continued basic disagreement as to how much more a healthy and wealthy West Germany should do to help boost the rest of the world's economy.

Underneath these concerns is uncertainty here about President Carter personally and his policies.

The most critical issue is the nuclear question.

Carter believes that the sale of nuclear reprocessing and enrichment plants to Brazil, which has not signed the nonproliferation treaty, could lead to the manufacture of atomic weapons. The Germans argue that nobody complained officially when their deal was signed, that their controls are tighter than those in the treaty and that if the deal were broken the Brazilians would eventually get the plants with no strings whatsoever.

An apparently frigid meeting with State Department officials in Bonn last week did not budge the Germans. Yesterday, Genscher flew to Washington in a hastily arranged trip. Although the Foreign Ministry says the trip is just to establish contact with the new administration, the nuclear deal - which means perhaps 20,000 jobs here - is a primary reason.

Today, Defense Minister Georg Leber flew to Washington to meet with the new U.S Defense Secretary. West Germany is incensed at what they view as misleading statements given to the press in United States by the U.S Army about the quality of the new German Leopard-2 tank in comparison to a new U.S. model.

There is alos strong pressure on Leber within the ruling Social Democratic Party not to give in to U.S. pressure to buy the AWACS warning plane. West Germany will have to pay about one-sixth of the $2.5 billion cost of 27 planes.