What do I do if my baby will not eat solid food?

Keep trying. Remember that, essentially for the whole first year of life, the bulk of nutrition and diet is contained in the breast milk and/or formula. Baby food is added to ease baby in transition from a baby diet to an adult diet - essentially to LEARN HOW to eat. That is why it progresses from stage 1 to 2 and so on. There is no amount babies MUST eat, so if they balk at it, just keep trying.
Patient persistence. Each infant has his/her own temperament. Infants that are slow to eat solids often have "slow to warm" temperament. That is the are slow to accept change or new experiences, like new foods, new clothes, bathing, etc. If they are slow to accept solids, don't force it. Just patiently continue to try various foods. Once one is accepted then mix it with the next food you are trying to introduce.