What kind of watering has it been getting. In warm, dry weather, it may need watering every 2-3 days, and possibly more often if the pot is on the small side.
Another thing to consider is the nutrients. In pots, these plants need regular feeding with an ericaceous plant food, according to the directions on the box.
Other problems are possible, but without further information, these would be my top two suspects.

I'd say it's probably too small a pot and too dry and totally agree about watering every 2-3 days.
Don't use a watering can or you'll be making dozens of journeys, use a hosepipe and put it on a medium speed and just leave it on the pot.

Just as a footnote here ....
You should have left at least an inch at the top of the pot which is soil free, this is so that you can add water the plant - that is you leave water from the top of the pot to the top of the soil to drain down through.
You then repeat this a second time and by that point the plant is supposed to have had enough water.
This relies on the gardener leaving ideally 2" soil free at the top.