HPLC pressure increasing


Usually I had pressure on 40 bar (+/- 2). But, recently I forced with a problem of pressure increasing after 20 minutes the pressure increasing up to 55 and then decrease. We analyze mixture of oil and phosphatides .it occurs in both cases injection of a sample and blank. Have you ever forced with such a problem? What could be the reason ? 

Technical characteristics:

Pump : Quat Pump VL (G7111A)

Detector: 1260 infinity || ELSD (G4260B)

Column: 250 X 4.6 mml S-5  μm.

We use gradient with 3 phases:

A: hexane

B: Methanol, Acetic acid, Triethylamine 

C: Acetone,Acetic acid, Triethylamine 

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