The game has been structured in such a way that each of the different aspects of your factory are equally important. To ensure the smooth running of the overal production it is recommended that you level everything up evenly.
If you neglect a part of the production flow you will end up creating a 'bottleneck' along the line which will slow the whole of production down. It is also equally important regardless of what you are trying to produce that you make sure that each workstation has another workstation making the required materials otherwise you will have workers just sitting idle doing nothing due to lack of supply.
Top Tip: Try to level up the warehouse and market in intervals of 10 because after every tenth level for both of these areas you will be given an additional transporter.