Spinach likes to be cool and it flowers (bolts) when the days get longer which is something you don't want to happen. It's difficult to find a consensus on how long the days need to be before they bolt and it depends on the cultivar also, some are slower to bolt than others. I would not to exceed 11-12 hours of light. Some plants flower based on the length of day, those that flower when the days are long are called long-day plants. Spinach is a long-day plant hence you need to keep the hours of light below the number that will cause it to flower.
A temperature around 60 degrees F should be OK for spinach, so I was wondering if there isn't enough of a drop in temperature at night. Alternatively, a plant's growth will be limited by whatever nutrient is in short supply which is why I asked about fertilizer analysis.
