The dose can be extended to 5 days, so I'd probably do that - from recollection, it's the same dose for days 2 & 3 for a further two days. Sometimes, it's quite persistent and/or the medication can take a while to take effect, but I'd definitely try this.
What are your pH, gH and kH? If you don't have tests, you can get the pH and hardness from your water supplier's website. Look for the page on water quality and type in your postcode, then let us know what the report shows.
Your water is quite hard, and mollies do require hard water. Yours is at the lower end of their required range but I don't expect this to be the problem - if anything, it's more likely to affect your neons which require soft water. Therefore, this aspect is unlikely to be the cause.
What are your readings for ammonia, nitrite and nitrates? How often do you do water changes and what proportion of water do you change at a time? How old is your molly? What other fish do you have in the tank, and are any of them showing odd behaviour?
Sorry for all the questions but hopefully, between us all, we'll get to the root of this to try to help your molly.
- My aquarium is 125lts and I currently have 5 mollies (3 fully grown and 2 younger ones) 7 neon tetras, 1 betta, 2 panda corries and a bristlenose pleco. The rest of the fish seem healthy. - I bought my molly a bit less than a year ago. She was actually pregnant when I got her and I kept 2 of her young ones. - I do water changes once per week (15 liters or so). - Current water readings are: 0.25ppm ammonia, 0 nitrites, 40ppm nitrates
That eliminates another theory I had - some of my own fish lost function of their swimbladder as they approached the end of their lifespan (age 5+) but your molly is still quite young.
From your next scheduled water change, I would increase it to double what you're doing which would be 25% water change weekly. Hopefully that will get ammonia down to 0, keep nitrite at 0, reduce nitrates, and dilute other things that we can't measure but which may be present and contributory (eg any stress hormones). Ammonia and nitrite should be at 0 and nitrates no more than 20ppm above whatever tap water nitrates are, so worth doing a test on tap water nitrate to find out what it is. It might just be that larger weekly water changes resolve the problem (and that what this molly is suffering from is long-term exposure to a low level of ammonia, with her being the weakest fish in the tank and therefore first to be affected). Let us know how things are a few days after the first of these larger water changes as an improvement might even occur as soon as that.