When defining a new NeutronSQ you can click to define the isotopologue. You can click +add to add in all the species, however if you click on one of the species (and highlight it) before adding in all the species, a species will always be highlighted thus preventing more species being added. The workaround is just to delete the NeutronSQ and remake it and make sure all species are added before clicking on any of them. (Using Dissolve 0.8.2 on IDAaaS)
This has been reworked and is fixed in 0.9.0. The patch will also be rolled in to 0.8.3, coming in the next week.