General advice

I'd wager that first plant is not as robust as the others as far as handling the climate. Can't see RH, but 19C is a bit on cold side. So, i'd wager RH is not controlled either. if in soil you supplement the soil - soils are constituted in a wide variety of ways. if it needed Ca before, it still needs more Ca now. How much you provide or what is provided in the soil as a whole can impact how often you add it, of course. (mulder's chart) if in soilless, it should have a well-balanced diet, including Ca, at all times with 10% runoff to avoid buildup over time. in addition to what you tap water adds, you probably need to provide 100 ppm or more of Ca to avoid deficiencies. Your choice in nutrient ratios might impact that, too. there are antagonistic and stimulative relationships between the nutrient molecules as far as availability to the plant. Allow resulting node spacing to dictate light intensity. Once you have it growing properly, take a klux or ppf measurement -- really doesn't matter which. a "par" meter on a phone is just converting klux anyway... same reason a TDS probe does not actually measure ppm. it measured electrical conductivity then converts it with one of several potential conversion factors to give a ppm guesstimate. the face there are several different conversiosn factors used should tell you just how accurate that conversion is, lol. With a klux conversion it's similar because not all LED grow light shave the same properties of light, which need a slightyl different factor to accurately translate lux/lumens to umol/s PAR. Don't over-think watering. 1) fully saturate 2) wait for appropriate dryback and repeat Don't make it more complicated than that. How long you wait depends on how fast the plant drinks. You shouldn't have a tiny plant in a large pot. You just end up with stagnating water and that's going to eventually lead to problems at some point that are 100% avoidable. A high water capacity soil, wait for top 1" to dry. In something that doesn't have a high water capacity, like coco, you re-irrigate much sooner than the top 1" dry -- what does stay consistent is the 'dry' weight for the most part. You can also learn a good trigger for irrigation by the weight of the pot. Lost mass is almost 100% lost water-weight, so it strongly correlates. If the simple act of fully irrigating causes any droop, that's just a shittily composed substrate. Use more perlite or similar next time. Basic watering should never cause a problem. Half-assed watering that doesn't fully saturate the medium is a very bad practice to repeat over time.