Short answer: you are using the wrong technique.
Long answer: The light travels from the LED strip in straight lines through the acryllic and emerges at the top of the acryllic plate. In order to illuminate the pattern, the light beams have to be deflected into your eyes. Engraving the surface does just that: it puts tiny scratches into the surface that deflect the light beams hitting it.
To get a very uniform deflection, all the scratches have to have the same orientation. You can see that the bottom edge of the engraving is illuminated very well, while the left and right borders are almost invisible. You probably turned the acryllic sheet around while engraving the border and now the places that the tool cut horizontally deflect the light outward to you, but the places where the tool cut vertically or diagonally deflect the light into different directions, thus making it less visible to you. You can see that very well in the D of "Legends".
The solution: You need to engrave the whole motif with a fixed orientation of the acryllic sheet and your tool. It seems to produce the best result if the tool cuts parrallel to the bottom edge, so the handle must be perpendicular to the bottom edge. You must not turn the sheet around or hold the tool in a different angle, regardless of the orientation of the line you're currently engraving.
An alternative solution is to order a laser engraving of your motif. Acryllic sheets are known to engrave well and create the desired effect. There are lots of companies online that offer laser engraving, but it's worth comparing prices.