As Box says, a model would help us help you.
Maybe it’s a ‘size’ issue.
If your object has tiny edges these might not get processed successfully - SketchUp thinks that two points within 1/1000" are coincident and skips them !
Alternatively - your object must be a ‘manifold solid’ - so when its grouped what does Entity Info say - if the header doesn’t include the word ‘Solid’ then it’s not suitable for 3d-printing, without some fixing.
Also even it it is a solid, does it have any ‘reversed faces’ - if so fix them using the context-menu.
There are lots of posts explaining what constitutes a manifold solid, and tips on fixing them…
e.g. this one: Whats happening with this model upload to shapeways?
Do a forum search…