I recently had an engineered wood floor installed with glue. In a few spots the floor sinks. Meaning when I walk on it I can press the wood down. Some areas are worse than others. What is the problem and how can it be fixed?

This is a case of inadequate floor prep prior to the installation. There is no remedy that I know of other than to remove the flooring in those areas and attempt a repair.

This is not an easy solution. However, a full replacement is not either.

Barquois is right, it is poor floor prep. You've got low spots in the installation that is resulting in the deflection you're seeing. However, there are remedies other than removing any flooring. Try Dritac's engineered repair kit. Drill a couple holes, inject the adhesive, and you're good to go......check it out http://www.dritac.com/productsRS.php