Plants growing on a slope are more difficult to water than ones growing on a more horizontal surface. That’s because the water you apply (or that Mother Nature applies) tends to flow downhill rather than penetrating the soil. And the steeper the slope, the greater the runoff and the drier the conditions! So a poor plant on even a modest slope is constantly suffering from a lack of moisture!
One way of compensating for this is to create a watering basin around each plant. Just cut into the slope on its upward side, leaving a depression, and use part of the soil to create a small semicircular berm on the downward side. This basin will help the plant catch water from rain as well as from overhead watering, allowing it to percolate slowly into the soil. Just leave the basin in place permanently (you may need to rebuild the berm occasionally, depending on the type of soil) and this will give the plant a much greater share of rainfall and irrigation.
Another possibility for watering a slope is to use a soaker hose or drip irrigation, running the hose across the slope, that is, in the at right angles to the slope. Soaker hose and drip irrigation apply water more slowly and in smaller doses than other methods, so it is much more likely to sink in and reach the plant’s roots than to flow down the slope, out of reach its reach.
Also, in the future, remember that slope plantings benefit less from rainfall and overhead irrigation than other plantings and will need to be irrigated more frequently.
Of course, the easiest way of successfully planting on a slope over the long term is to only use drought-tolerant plants in such places. They’ll still need some watering for at least the first year, but, as they root in and establish themselves, should soon be able to take care of themselves.