Modern birds have had their front legs, or what might be called their "arms", converted into wings.
These wings are obviously essential to those birds which need to fly.

But flying is not something all birds do. Some spend all their time walking about on the ground - ostrich, kakapo, cassowary...

Could these encumbrances be converted back into useful arms?
A pair of manipulative arms, would surely be a much better survival asset, than a pair of almost redundant wings.

Is there any sign that ostrich, or any other flightless birds, are evolving arms again - if not, what stops them from doing it?

That is a simple question with a very complicated answer.

To be brief, the answer is "maybe".

The possible evolution of a particular characteristic is always subject to caveats and probabilities, because evolution is itself a stochastic non-directed process. Birds, like mammals and reptiles are tetrapods. All four of our limbs were once fins, then legs, then some became arms and wings, and in some cases these became fins again. Taking the example of the cetaceans, where limbs were once again converted into fins, and also the case of aquatic birds where a similar process has occurred, you can see that changing shape and function of limbs is of course possible.

However, when considering such changes it is important to remember that evolution is contextual, forever building on the innovations that came before it. What this means is that if flightless birds (and these are by no means a unified group) were to have their wings altered over generations, there is no reason that they would resemble the grasping hands and arms that their non-avian dinosaur ancestors possessed. Instead, with different genetics and different environmental pressures, I have no doubt that an adequate but different solution would arise.

At one extreme, the kakapo has a full wing, but it's flight muscles are underdeveloped. Then there is the Kiwi, whose wings are tiny reduced nubs and it's feathers are more like downy fur then a flight surface. Both birds are flightless, but I suspect that given the same selection pressures you would have two very different outcomes.

You said "A pair of manipulative arms, would surely be a much better survival asset, than a pair of almost redundant wings."

Again, that entirely depends on the circumstances, and the cost involved in having four active limbs over two. The cost to grow and maintain large muscles and bones is always a consideration. Small, redundant wings might be less useful in certain ways, but they cost much less energy and can still be useful, such as in mating displays, maintaining balance, or keeping warm.