If you mean the bloom post-process effect, it means your emissive output is not strong enough to reach the bloom threshold by itself. Add a multiply in your shader just before plugging into emissive output to crank it up. The value you want is probably somewhere between 1.5 and 4.0.
Alternatively, lower your bloom threshold -- however, this means bright surfaces in general are more likely to bloom.
Sorry guys, didn't notice there were already replies. What I meant was the emissive, they stop glowing once they're not hit by light. But I already figured it out. Thanks for the help.
nicocabbalero: Well, I really don't know how actually. But I figured out that I was having that problem when I use a dominant directional light, so I just used a point light instead.
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Alternatively, lower your bloom threshold -- however, this means bright surfaces in general are more likely to bloom.