Was my Photodynamic Therapy done incorrectly?

In our experience, there are a couple issues that may affect what kind of response you receive from the treatment:

The incubation period - the amount of time spent with the Levulan (amino levulanic acid or ALA) on the face. This can vary from 60 minutes to several hours and is determined by the practitioner.
The device used to activate the ALA. A pulsed dye laser will generally result in a more dramatic response than an IPL or Blu Light device, but the device used depends on skin type.
The way in which the face was prepared and the care taken to apply the ALA. The practitioner needs to take care to carefully clean and prepare the face before application of the ALA. This combined with proper application of the product will result in maximum absorption of the ALA and a more aggressive response.

The outcome of a Photodynamic Therapy (PDT)  treatment can depend on several variables : the  technique of the procedure,  the tool used  ( light source) and the degree of correction needed ( amount of skin damage , acne scarring and age).   Studies have shown that the application of the of the aminolevulinic acid  ( ALA) is best absorbed when applied to  "prepped" skin surfaces for an incubation period of 60-70 minutes.  The use of an IPL laser lends the advantage of not only activating the medication for a dramatic effect but treating the redness and uneven discolorations .  The Sciton laser does this effectively!

Clinical outcome from photodynamic therapy (PDT) for actinic keratoses or acne unfortunately is not an exact science. Important factors include incubation time (amount of time Levulan or Metvixia is left on treated area), whether or not occlusion was utilized (such as on the scalp or back), pre-treatment protocol using topical retinoid or 5-flurouracil, and whether or not adequate pre-op cleansing was performed. On top of that, I have recognized a skin hardening effect after having supervised hundreds of PDTs, where it may be necessary to increase incubation time to bring out similar inflammatory response as the prior PDT treatments.

In our experience, patients can have different reactions to PDT even when administered in the same office by the same person.  Patients who have undergone prior PDT in the same area usually have a less aggressive response the second time around.  

It is not uncommon to have less of a reaction as you have more treatments as there are fewer actinic keratoses on the face in the area.  If the incubation time was shorter the third time, that could explain a kinder reaction as well.  The time under the light could also affect the treatment outcome.  I would suggest getting your records from the previous treatment, so your current physician knows how long you were incubated and how long you were under the light.

It is not surprising that you had a decreased reaction on the third treatment. If the incubation periods were the same, it is likely that you have less pre-cancerous areas on the face.  These were probably treated after the first two treatments (typically it takes 2 for good effect) and that is why you had a decreased reaction on the third time.

Obviously something was different - perhaps the concentration of the levulan or the settings on the IPL. See if you can find out what was done previously and let your doctor know.