Despite it showing 6 slots, you're actually capped to however many fragment slots your aspects give you. On Solar Titan, the cap will be 4 as each aspect only gives access to 2 slots. In slots 5 and 6, you'll see the fragments padlocked (the ones you have equipped in slots 1-4 won't have a padlock) and it'll say 'Not Enough Energy' as you don't have slots 5 and 6 available.