6 Anchors and 2 tags

I put three Anchors around the 2m*3m competition venue。
with height of 0.35m
And the tag is on the robot with height 0.35m。The picture below is one of the 3 Anchors

The test video is with only 3 anchors and 1 tag in channel_7.

It works great.

Then I am trying to implement 6 Anchors 2 tags system. 3 Anchors and 1 tag are all in channel_7 and the other set of 3 Anchors and 1 tag are all in channel_4. Both tags works well with around 3 minutes. Then one set of the tag starts not to receive one of the 3 Anchors in same channel.
Below picture is the two Anchors. I put 2 Anchors in same point.

(0,0) (0,0)
(2,0) (2,0)
(1,3) (1,3)
with height of 0.35m and 0.3m
I think maybe the problem is I put two anchors too close? And the electromagnetic wave is too strong, so it interrupt?