Nitro lacquer, technically speaking, does not actually 'cure.' It is strictly evaporative, meaning that when all the solvent has degassed it is as hard as it going to get. Think of a pot of boiled pasta then imagine all the water evaporating leaving behind a dried shrunken layer of pasta - that's essentially what nitro lacquer is.
As opposed to reactive finishes, where the finish molecules continue to cross link together for quite some time even after the solvent/vehicle is gone.
Reactive finishes leave witness lines when you sand through the layers (each layer remaining physically distinct from the others), evaporative finishes (lacquer, shellac) do not - each freshly applied layer slightly dissolving the previous layer and melding together to form one thicker layer.
I cannot see how a sealer underneath the topcoat would retard this process. The only possibility I can think of is if there were plasticizers in the lacquer and somehow these interacted with the sealer to become more 'potent' thereby keeping the final product softer than intended. Although. given the product they are selling - with the full knowledge that many DIYers are going to use nitro as a final finish, I seriously doubt the people at Warmoth would choose a sealer that was not nitro compatible.
As mentioned temperature and relative humidity all affect the time it takes for degassing to occur. Cold and damp conditions could delay that process for quite an extended period.
If you have an empty closet in your home try hanging the piece in there with an incandescent bulb left on. This often can provide the warmer/drier micro climate necessary to get full degassing. Use an empty closet unless you like the smell of lacquer thinner on your clothes.
Exactly what product did you use?