This question does not provide much useful information about the equipment for fluid properties, so the best answer is to look carefully at the system.

1.    In a large storage tank, it may take more than 3-4 hours to break a strong density or viscosity layer.

2.    Nozzle velocity, diameter, and flow rate may not be sufficient to reach across the tank and promote circulation.

3.    An inline mixer does not necessarily maintain viscosity unless the flow velocity is sufficient to blend different viscosities
        a.    static inline mixers are not good at blending different viscosities
        b.    a dynamic inline mixer should be capable of blending liquids

4.    Layering in a storage tank should be avoided by the location of liquid addition:
        a.    light liquid should be added near the bottom
        b.    heavy liquid should be added on the top
        c.    liquids should be combined before going through the jet nozzle.

5.    Are the layering measurements accurate or are local differences being measured.

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