Some of the net spend can be explained by the sharp increase in transfer value. The buy Rodri and a year later he goes to City for triple the price. Barca and Bayern triggered the release clauses of Griezmann and Lucas Hernandez. The sells can be attributed to a few players but they really buy some duds.
Players that cost 20m or more since 2012: Morata (50m), Joao Felix (113m), Marcus Llorente, Mario Hermoso, Thomas Lemar (63m), Rodri, Gelson Martins, Diego Costa (59m, the current one), Vitolo, Kevin Gameiro, Gaitan, Jackson Martinez (managed to offload him to China), Savic, Carrasco, Griezmann, Falcao.
Savic was fine, Falcao, Griezmann and Rodri were great and the rest have been duds. They've fetched a great price for some of their players but recruitment is poor when they are desperate to fill in a position. All of their successes are players that have gotten a lot of time. Home grown players and players who were bought young (basically home grown). I don't know what quality they have in their youth team so unless something comes from there I don't see another trophy in the near future form them.