I had a similar problem. The X6100 is sensitive to SWR and will reduce power a lot if SWR is over 2.
To be 100% sure the ATU is locked in do the following.
Make sure TX CQ and TX call are disabled.
Long press ATU.
When ATU is finished reenable TX CQ and TX Call.
If the antenna is still out of range of the ATU you will not get the full 5 Watts.
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Hope this helps.
