If you're having charging issues, please make sure you seat the transmitter completely into the receiver, otherwise the transmitter will not charge. For more details, see this article: Relay G10 FAQ
We have designed the G10 transmitter to fit into almost any guitar. However, we anticipate that there will be some rare cases where the transmitter does not fit into a given guitar. If the G10 transmitter is inserted into your guitar and the transmitter's actuator pin is not engaged, the transmitter will not turn on, and the LED on the transmitter will not light up.
In some rare cases, the guitar’s active electronics are wired with inverse polarity. In this scenario, the transmitter's electronics can go into a protection mode that keeps the system from being functional. This is evidenced by no output from the G10, even though the transmitter's light is on.
We have found that a workaround to both of these issues is to use a mono-to-mono (TS-TS) Male to Female adaptor. More details and a troubleshooting procedure can be found at http://line6.com/support/page/kb/_/live-sound/relay-digital-wireless/relay-g10-guitar-compatibility-fitelectronics-r817.
If these issues affect you, we encourage you to post the following information so that we can better understand the scope of the issue and the effectiveness of the solution:
Guitar manufacturer
Guitar model
Does it fit and turn on?
Do you hear audio?
Does the adaptor solution resolve the issue?