Try the following if the flash takes time to charge
1. Check the battery level
The following icon indicates that the batteries need to be replaced.
  Low Battery Power Indicator When battery power is low, the icon above appears on the LCD and the Speedlight stops functioning. Replace or recharge the batteries. | 
2. Replace the batteries with four fresh AA batteries that are all the same type
If the flash-ready lamp takes more than about half a minute to light after being fired or turned on, you will need to charge the batteries (rechargeable batteries only) or replace them with fresh batteries. We recommend using 1.5 V AA alkaline or rechargeable 1.2 V AA NiMH batteries. Replace the batteries using four fresh or newly-charged batteries that are all the same make. Do not mix batteries with different charge levels or of different makes. For information on supported battery types, see the documentation for the flash unit.
3. Check the minimum recycling time and number of flashes for batteries of different types
The performance of alkaline batteries varies widely by make. 1.5 V AA manganese batteries are not recommended. External power sources (those available from Nikon are recommended) offer a reliable power supply while increasing the number of flashes and reducing the recycling time.
 
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The instructions above may not apply or may contain features that are different from your product.
For more details about your product, please refer to the documentation in our Download Center.
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■ If the problem is not resolved
If the instructions above did not solve your problem, please contact Customer Service.
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■ If repair may be required
If you suspect a malfunction, please send your product in for repair inspection as soon as possible.
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■  Information for Inspection Service
To maintain optimal performance of your device, we recommend that you have it serviced at regular intervals depending on usage.
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