I just picked up a Slim Phatty and I can’t seem to get the Arpp to latch. I did what the manual says and toggle between A and L with the enter button but still no latch? It just plays thru one cycle from high to low and stops. Help. 
Advanced preset menu->arp-> switch it from once to loop or back-forth
Did it and still not latch. Arhhhhh 
When the “A” appears on the display, saying the arp is on, press Enter, The A should turn into an L.
Have you consulted the manual yet? It might be more precise that any of us here.