Predictive modelling of a series of JET plasmas with both edge (ETB) and core (ITB) transport barriers has been done using the transport code JETTO with empirical or theory based transport models. Three specific experimental situations, which present a significant challenge to modern transport models, have been selected and analysed. The first one, recently found on JET, is optimised shear plasma with double ITBs [1]. The second experiment deals with stationary ELM-free H-modes, reported by many tokamaks, including JET [2,3]. Finally, we look at recent attempts to reach type-I ELMy H-mode above Greenwald density limit without degradation in plasma performance [4].