This was an old problem that was more common when I first started painting glow stars about 15 years ago. Every now and then I had a frustrated customer wondering why she had put her stars in daylight all day long but they weren't glowing. These were also usually people who had not yet put the stars on the ceiling but had them sitting on a table. So some of these people were not looking at them at night with their eyes adjusted to the darkness. Not the best way to view glow stars. 

It's also not the best way to charge the stars. It's less of a glow problem and more a timing problem. 

If your stars are in daylight all day and sunset is around 5 or 6pm, bedtime is not usually until 9pm or 10 pm. That's a gap of about 4 or 5 hours where your stars have been glowing, they have been releasing stored light all that time and not being charged. You won't be seeing them at their best. It's a bit like wondering why your coffee is cold four hours after you made it.

For that reason the instructions say use a top up at night with a bright light. It should only take about 10 minutes right before bedtime. Daylight is sometimes enough in some rooms but the top up with a bright light before bedtime ensures that the stars are as charged and as bright as they can be.

Back then I used to recommend using a bright fluorescent light, compact fluorescent light, bright flashlight or floor lamp as a top up and making sure the light was as direct as possible.

Now LED UV lights are sold cheaply everywhere online, most of them are super bright and these are the fastest and strongest way to charge the stars. It only takes a few minutes before bedtime. 

If your stars are not glowing bright enough with your standard room light use UV light, too easy!  

I recommend using a 395 nm flood light or bar, 30 - 100 watts is bright enough to charge a star ceiling in a few minutes. 

This year I've also started including stars of other colours like red, orange and violet in the star stickers packs. These especially need a good UV charge right before bedtime as they have a much shorter glow time and very short colour glow time. Violet/pink is a colour that will almost never be seen at night without a UV light. Leaving these new coloured stars in the sun all day will do absolutely nothing as they will be completely dark by bedtime. The new stars will glow faintly on the ceiling with standard lighting but as background stars and only for about 3 hours. You won't see much colour without a UV light. 

Most people are savvy about glow stars now, they already have UV torches ready and charge glow stuff easy but I still get the random "I charge my stars all day in sunlight and they don't glow, I need to return them they're faulty" email...and just like that I'm back in 2011.  

To sum up; follow the simple instructions, use daylight and/or a top up from a bright light before bedtime. UV lights are best. :)