The vast majority of wall dimmers work well near their rated load and down to about 20 or 30% of it. Below that, they tend to "cut off".
Increasing the load to be closer to rated capacity improves the low end range. However, LEDs are so amazingly efficient, they are nowhere near the dimmer's capacity.
You have three basic choices:
- Add some load along with the LEDs, like maybe a 60 watt incandescent, or a lot more LEDs. That will get the dimmer load closer to its design parameters. Alas, it also defeats the whole point of energy efficient lighting.
- Upgrade the dimmer to a more suitable technology. There are dimmers specifically made for LED control, like Lutron's C-L line but they may be hard to find. Here is one at Home Depot.
- Replace the dimmer with a lower wattage capacity in the same technology. This gets the dimmer closer to it designed range. But a cheap one less than 600 watts is hard to find and I bet all your LEDs total less than 50 watts.