That sounds a little dry to me, and the fluid is pretty viscuous even with more water. You have two different layers of fluid? One purple, on yellow?
Stir, man. The heat it up to at least 120C, somewhere along the way you will get that sudden surge of action when the I2 and phos acid react to make the rest of the HI. I wonder with the small amount of water you had and heating it
you oxidized you phosphorous acid to phosphoric and have nothing to recycle the I2. Got a little red phos? Toss some in and stir. Might surprise you what happens. If the reaction gets active and goes yellow, you can bet you did not have the phos acid in the right molar amount to sustain recycling during the reaction.
How are you going to get a reaction to complete if your ingredients are not homogeneous?