Ming Rider - uneven development and/or streaking are often the result of insufficient or non-agitation. The reason is that as development proceeds, by-products of development accumulate. These by-product compounds come from both the developer and the film. Depending on the type of film and the composition of the developer, accumulated development by-products can have a variety of effects. For example in some cases these compounds slow down development locally. One of the functions of agitation is to remove these by-products from the emulsion surface. When agitation is insufficient the by-products will tend to simply accumulate and flow downward along the emulsion surface causing streaks of restrained development or broader areas of unevenness. Note this is just one simplified example to illustrate without getting into the specifics of certain developing agents etc. There are many variables. Different developers and films will respond in different ways to reduced or non-agitation. Some combinations work better than others.