Demand side flexibility is crucial for matching supply and demand in the future as power and energy balancing of the system is likely to become a difficult task using conventional tools in scenarios with high penetration of nenewable energy sources. Aggregators are meant to contribute to the increase of the share of demand side flexibility provided by relatively small residential and commercial resources. In particular, distribution system operators may benefit from the activation of such flexibility in order to alleviate congestions in load areas operated by them. A specific flexibility product is studied in this paper in which the capacity for a specified generation/demand modification is previously procured for a specific period of time but the activation is optionally triggered by a control signal from the flexibility buyer at short notice. When delivering flexibility, aggregators need to have tools to correct bad performance of flexibility resources due to technical issues arising in real-time domain in order to remain qualified players for flexibility trading. In this paper, an algorithm to allocate flexibility volumes among available resources connected to the same load area is developed. The algorithm corrects possible failures to deliver by means of the verification of energy measurements from the aggregator's resources portfolio. The aim of this design is to help increase the efficiency of flexibility delivery from residential network users and contribute to a better utilization of their resources. The algorithm is implemented in MATLAB and validated in a case study where energy measurements deviated from expected values are artificially generated.