Why isn't just one session of subcision enough for acne scars? Aren´t all the fibrotic strands broken with the needle?

Excellent question! Scar pathology and histology is complex. In some cases only 1-2 sessions are required, in others 2-5 sessions. The reason is the complexity of attachments to fibrous septa, SMAS layer and dermal scarring. Think of a scar like branches of a tree. To break all bond in one session 3 dimensionally is difficult in complex scars. Hence multiple treatments from all angles. The ideal treatment for acne scars will depend on the morphology of individual scars. For example, rolling scars are best treated with subcision, atrophic scars with fillers, pick-ice pick scars with TCA or Phenol CROSS/ Paint, box-scar scars with fractional laser/ modified TCA/ phenol chemical peels, hypertrophic scars with steroid injections/modulated fractional delivery. 
Energy devices such as lasers, microneedling RF and bipolar radiofrequency are other methods to treat ‘general’ scar types. For example devices can improve rolling-tethered scars as well as ‘deep - superficial scars’ such as broad ice-pick scars and broad box-car scars. Devices should be viewed as a method of scar revision, and rarely as a sole modality. Exceptions do apply- for example in patients with predominantly shallow/broad box-car scars and or superficial pits. 
Another factor that will determine the ideal combination of treatment is the skin type of patients. Skin typing used to determine the ‘skin colour’ of patients. Darker skin patients will require modifications to treatment parameters. This includes a lower percentage of chemical peels, conservative protocols for lasers and energy devices and in some cases pre and post treatment of skin to reduce but not eliminate the chances of PIH- post inflammatory hyperpigmentation- skin darkening. 
The very first step in scar revision is to accurately determine the subset of scars you have. This is best done with a real time examination. Factors such as animation (rather than static photographs) are frequently used to determine the amount of ‘deep scarring’ /anchored scars or tethering. Palpation or touch is also extremely important to determine the presence of dermal and subdermal fibrosis. Scars that have altered ‘texture’ either hard or soft (atrophy) are more challenging to treat compared to scars with ‘normal’ texture. In some cases the use of angled lighting can be employed to accurately map scar types. 
My answers given in the context of this question is a general plan given the limited information. For an accurate guide, one has to take into consideration factors such as animation, touch, feel, real time examination and most importantly subjective measures, namely patient expectations. A real time consult with a procedural dermatologist or plastic surgeon is required for the best accuracy- diagnosis.