Ryobi 36V Battery reads fully charged but won’t work in my tool

Firstly, put the battery in another tool to check that the fault lies with the battery and not the tool.
Next, check that the terminals on your battery and tool are free from any debris or dirt. If you do find that your battery or tool needs to be cleaned, use a tissue to clear any debris.
Ensure that your battery is not too hot. The operating temperature for batteries is below 45℃. Before using your battery, ensure that it is cool to the touch.
If the issue persists and your product is still within the warranty period, take it back to the retail store along with your receipt or proof of purchase for warranty assessment.