Printed standoffs won't stick

[w9ran]

I must be doing something wrong because the bosses or standoffs for screws I'm trying to print on the bottom of a "box" won't stick to the printed surface - all ll I get is a mess of filament. Top image shows the 3D design (Tinkercad) and the bottom shows the result after printing.

To design this I first created the box, then placed a workplane on the inner surface of the box and placed the standoffs on it and grouped them all together. When I look at the finished print closely there is no indication of the standoffs at all, like it tried printing on air rather than on the surface. Is it necessary to revert back to the original workplane and "sink" the standoffs into it? I thought I'd ask for advice rather than just trying random stuff as this is clearly not a difficult thing to do, thanks!

[LePaul]

What does it look like in the slicer preview?

[L67GS]

Bad stl file? Maybe try a file that has a similar object and see if it still happens.

[Roberts_Clif]

I would revert back to the original workplane and "sink" the standoffs into it?

[LePaul]

If you can, share the STL with is? I am curious of those standoffs are part of the lower layer, as Cliff points out