Leds blink with a different power supply
The datasheet you have linked in the comments (it should be in the question) is for "LMS1585A 5A/LMS1587 5A and 3A Low Dropout Fast Response Regulators". At best this can give out 5 A.



Figure 1. The VI curves for the LEDs. You're using the 385 nm (red curve).

Your LEDs are going to drop 3.4 V at 0.5 A. That means your resistors will drop about 8.6 V. (We'll home in on a closer value later.) The current through each resistor is \$ I = \frac {V}{R} = \frac {8.6}{27} = 0.32 \ \text A \$. With 20 LEDs you will need a supply capable of 6.4 A. The voltage regulator is dropping out on over-current or over-temperature.

Returning to the graph we can see that at 0.32 A the forward voltage is a little lower so we could recalculate if we wanted but it's not going to make much difference.

Your circuit is not efficient since 1/3 of the power is used by the LEDs and 2/3 wasted as heat in the resistors. Instead you can put two or three LEDs in series on each branch. Recalculate the resistor values for your chosen layout. With three in each branch you will require seven branches and a total of 7 x 0.32 A = 2.24 A. Close!

I have written about using a Loadline resistance graphic tool and it may be of interest.