I ganged two 12 gallon reservoirs together and ran four off of that setup: plant trays filled nicely although number 4 didn’t even start until number 1 and 2 filled.
Mine do that too at first set-up or rez clean, probably just an air bubble in the 3/8 leader line. Thankfully the plants dont throw back like a college kid on a beer bong, so once full not an issue. Watch for dribbles at the spout where the line goes into the unit. I drop a small piece of papertowel at each one and watch it, if it wets the valve needs to adjusted/leveled or the silicone bumper isn’t sealing.
I have two 25 gallon rezervoirs tied together for when I’m going full steam ahead but I haven’t used one in like a year… Because you know my working away and all.