Sometimes plants purchased from garden centres are in a fine potting mix that can dry out quite quickly and this kills the roots. Re-watering gets the soil moist again, but if the roots are toast then its an uphill battle as to whether it will survive no matter how much water they get. I'd try re-potting them with garden soil from your garden centre and see how you go, it might hold the moisture better and prevent a terminal dry condition again.
You can also try a 50:50 mix of potting mix and mulch, of which you can buy 40L bags for $5 or so. The potting mix is designed to not break down over time in pots while the mulch is, so you're getting the longevity plus the moisture-holding from the mixture.