Leaves turning yellow could be a sign of too much water or poor drainage. Mushroom compost tends to hole lots of moisture so my first guess is overwatering - especially if the cukes are in a container with poor drainage. The fact that they grew fine and then had a problem also suggests the soil became water logged over time. Dig into the soil down six inches and see if it is wet. The soil should be just barely moist, not wet. In the future most vegetables like a soil that drains quickly but can hold moisture so perhaps 3 parts soil and one part mushroom compost. Keep growing, Marianne Binetti