If the brakes are weak, then things most-likely are not adjusted correctly. They typically lock up around 50-60% as indicated by the LED strings. For a mini, they even lock up sooner because the plane is a lot lighter than the larger T-1s. My student's Mini locked up very quickly. But he did not have an LGC 15. So I can't tell you where that one locked up its brakes. But, if you are seeing all LEDs lite and the brakes are not locking up something may be installed incorrectly or, your battery voltage is not getting to the electro magnets due to poor connections, being to low, etc. If you are not seeing all the LEDs in the strings light, then the Travel Adjust in the transmitter may be set too low. You should see all LEDs in the two strings lit at 100% travel adjust. If you are seeing that and the brakes are still "weak" then you do not have the LGC programmed correctly. If everything is programmed correctly, they will lock up. Then you can adjusts them with the LGC or the Travel Adjust to still brake just short of locking up. I have mine on a slider. I just push the slider forward all the way and both planes come to a complete straight stop w/o any skidding or dancing, etc.
It takes a lot of playing around on the bench to figure out the programming. Trying to do it all at the field is a PITA. But once you get the LGC and Transmitter set right, the brakes work well. Also, once you get one set correctly, use the SD card and transfer the program to other planes with the LGC. I have found what works in one plane is a great starting place for another plane. Just have to work out the door sequencing, if applicable.