Over time, and particularly during hot and dry conditions, soil and potting mix can become water repellent (hydrophobic). This can result in water runoff or moisture pooling on the surface and not moving down into soil or potting mix evenly and effectively.
To know whether your soil has become water repellent, the next time you've thoroughly watered the garden, dig a hole in that area to a depth of about 20cm. If the soil is moist the entire depth, then water is being properly absorbed and getting to where it's needed most - the roots. If the soil is still dry underneath, then the soil has likely become water repellent.
With potted plants, in addition to water not soaking into the potting mix, water can also run down the gap between the potting mix and the pot wall, filling the saucer but leaving the root ball dry.
Use a soil wetter to break down the waxy, water-repellent layer on soil and potting mix and enable moisture to penetrate down into the root zone. Yates Waterwise Soil Wetters can be used on garden beds, lawns and potted plants to help fix water repellency and improve moisture levels in soil and potting mix.
Here's more information on How to Fix Hydrophobic/Water Repellent Soils.