The problem is jumpshots, plain and simple.
Bonner is useless if he isn't hitting 3's at am elite rate, and his current 36% average certainly isn't that. Compare it to his near 44% average over the past 4 years and you see the problem.
Diaw is having an even worse shooting season, hitting only 30% from 3. That drop (down from 40% last year) combined with him taking those shots more often (19% of his shots last year to 30% this year) is problematic.
Timmy is having his worst mid range shooting season in nearly a decade, hitting only 35% between 16ft and the 3 point line. This is a worrying trend as he was 37% last year from 40%+ the previous 6 years. He's taking that shot at a far lower rate, accounting for only 11% of his shots whereas is had accounted for over a quarter of his shots the past 3 years.
This Spurs team is based around pace and space, and none of the bigs are giving anyone space with reluctance to take or inability to make jumpshots. Combine that being 19th in the league in pace, mainly due to Parker being out, and you can see why we've struggled mightily on offence.