There are a few reasons that your purchases may not be going through.
Purchases may fail if your internet connection is weak. Please try again in an area with a good, stable internet connection.
You may have hit our daily spending limit. This spending limit is in place to prevent unwanted scenarios where a friend or family member may attempt many purchases in a row on your credit card! Simply try the purchase again the following day and it should go through just fine.
The device's payment restrictions may be turned on. You can check restrictions in your device's settings under General > Restrictions. If you want to turn on restrictions, please check out Apple's Restriction Guide.
Apple's payment troubleshooting information can be found here: https://support.apple.com/en-us/HT203005
Your payment method may have been declined. Please check your connected e-mail inbox for a message from Google regarding your last purchase. You can also view your purchase history and the status of the missing purchase by following these steps from Google Play: https://support.google.com/googleplay/answer/2850369?hl=en
Google's payment troubleshooting information can be found here: https://support.google.com/googleplay/answer/1267137?co=GENIE.Platform%3DAndroid&hl=en