The phrase "a combination of declining" is correct and usable in written English.
It can be used when discussing factors or elements that are decreasing or diminishing together in a particular context.
Example: "The report highlighted a combination of declining sales, customer satisfaction, and employee morale."
Alternatives: "a mix of decreasing" or "a blend of diminishing".
A combination of declining ad revenues, falling circulations and the growth of internet-based rivals have eaten into profits.
But that is more than counterbalanced by a combination of declining use of coal and reductions in energy demand from structural changes in the Chinese economy, with energy-guzzling heavy industries like cement and steel production both now declining.
Far more students attend our great public universities, where a combination of declining state support and unfavorable ballot measures pose a serious risk to our model of higher education.
And for months now a combination of declining press interest in the West and restrictive action against journalists has kept reporters, photographers and camera operators from reporting the familiar choreography of stonings, burning tires and gunfire that has sustained the uprising since it began in December 1987.
A combination of declining service and more (budget-driven) enforcement by the IRS in the mid 1990s led Congress in 1998 to overhaul the IRS, ordering 72 new taxpayer rights and protections.
In my opinion, each side is partially correct: the problem of rising public college tuition is driven by a combination of declining state subsidy levels and the inability of universities to exercise the cost discipline that is common for well-managed organizations outside academia.
Functional elastic fibers decline with aging and sun exposure as a result of a combination of declined synthesis, faulty assembly, and increased degradation, leading to skin fragility, sagging, loss of elasticity, and wrinkles.
Likely due to a combination of declines in incidence, massive scale-up of ART coverage [5,30,31] and rising coverage of PMTCT [32], the burden of HIV/AIDS has declined in the last half decade.
The substantial increase in deployment of customer-sited solar photovoltaics (PV) in the United States has been driven by a combination of steeply declining costs, financing innovations, and supportive policies.
The combination of declining real household incomes and a worsening degree of inequality combined to push up the incidence of official poverty problems by the end of the decade.
And the obvious answer is demography: Japan's combination of declining birth rate and lack of immigration apparently means a shrinking rather than growing labor force over the next several decades.
