The problems are abundant, but the most obvious problem with the Spurs at the moment is that their 4 best defensive players are Duncan, Splitter, Kawhi and Green..unfortunately for the Spurs, Splitter's offense has completely regressed, Kawhi's shot is broken and he can't play with Parker, and Green is just lost at the moment..
This forces the Spurs to play Manu(old, can't give you consistent defense) and Beli(horrible defender)..
Last year, the Spurs were getting solid offensive production from Green, Splitter and Leonard, which allowed the team to play high-level ball on both ends of the floor..