what is the difference

不一会儿 a few moments later/after a while  https://www.cchatty.com/words/127664

一会儿 a moment/a while/in a moment/now...now.../also pr.  https://www.cchatty.com/dict/17918

1. As a Noun  我午饭后睡了一会儿。
2. As an Adverb 

      in a while/an instant; presently; soon  坐吧，我一会儿就完。

       now ...now/then ...屋外一会儿出太阳，一会儿下雪。

1.表示时间的时候，都表示很短的时间。Both of them express a short time when referring to time. 
不一会儿，他就到了。     他一会儿就到。
2.一会儿还可以这样用，比如：他一会儿躺下，一会儿又来回走动，心神不宁的。

both are short time. but 不一会儿 usually is at the beginning of sentence.

mao qu bie