The phrase 'have not yet been successful' is correct and usable in written English.
You can use it when you want to describe a lack of success. For example: "We have been trying to attract new members to our organization for the last few months, but we have not yet been successful."
Adamatazky's early attempts at this have not yet been successful.
Alternative attempts to provide the consumer with visible cues in healthy products have not yet been successful.
Our results indicate that while fires do reduce stand density, they have not yet been successful in consistently restoring pine and oak dominance in the canopy.
We have not yet been successful in extending that quiet zone to a spouse, although someday the wife or husband of a future candidate will manage to live a private, apolitical life as Judith Steinberg Dean tried to do.
While Japanese companies have come to dominate the market for memory chips, which store information in computers, they have not yet been successful in chips such as microprocessors that manipulate information.
Recent attempts to qualify the LEU Mo) dispersion plate fuel with Si addition to the Al matrix up to high power and burn-up have not yet been successful due to unacceptable fuel plate swelling at a local burn-up above 60% 235U.
"In the past two years, my family has been under all sorts of financial and psychological pressure to find a way to bring her to the UK for an operation which can save her life and we haven't yet been successful," said her mother.
There are plenty of people who are smart and work hard who haven't yet been successful, while some of us were lucky in some way that contributed dramatically to our success.
Although the treatment of AIDS with highly active antiretroviral therapy has been successful, the development of a vaccine has not yet been successful.
The above data provide clear proof for the feasibility of anti-adhesion therapy of infectious diseases, although this has not yet been successful in humans.
