James Key believes Toro Rosso should be disappointed by their "really pathetic handful of points", saying the reliability is "simply not good enough."
11 races into this season, Jean-Eric Vergne and Daniil Kvyat have scored just 17 World Championship points with the team's best result being two P8s.
That has put Toro Rosso seventh in the Championship, nine points ahead of Lotus but a staggering 80 behind sixth-placed McLaren.
It's a showing that has disappointed technical director Key, who believes they have been let down by reliability.
"I think it's been a real mixed bag for us because we've got a really pathetic handful of points out of what should have been a much better position at this stage given that fundamentally we've got into Q3 on almost every occasion," Key told Autosport.
"All the issues have been completely different, which has been the big frustration.
"One area of weakness that you can focus on and get fixed quickly is great, [but] when you have a real random scattering of stuff you've never seen before, and it catches you out and then disappears again, it's really tough.
"Some of it definitely has been of our own making, but other issues have just been unfortunate.
"We did not finish eight times and this is simply not good enough.
"I think we have suffered from not doing a proper winter testing programme, because we are still observing new issues on the car which normally you sort out after testing.
"But we had a lot of problems during these sessions, and as a result we simply couldn't find out the weak points on the car during this very important preparation period."
The reliability issues have also hampered Toro Rosso's development with the Brit revealing they have had to slow it down to avoid more issues.
"We've been pushing probably the hardest the team has ever pushed, and our development rate has been quite reasonable.
"But when you're pushing a relatively small team to that level, inevitably mistakes can be made, or not everything is covered quite in the detail you'd want.
"So we've actually said: 'right, hang on, let's just step back and make sure that the fundamentals are right'."