Post a drawing sample (with just the element/s you are having problems with). Also, let us know what version of AutoCAD Architecture you are using. @David_W_Koch might be correct but it is my experience that if layer 0 is frozen, elements on layer defpoints disappear.
@David_W_Koch wrote:Do you have Layer 0 Frozen and/or turned Off? If so, make it thawed and turned on. Can you select the Defpoints items now?
This. Turning Layer 0 off will make any objects on layer Defpoints unselectable.
@dbroad : I tested it in AutoCAD Architecture 2020 before posting. Items on Defpoints remained visible, but unselectable, when Layer 0 was frozen, turned off or both. That does not mean that is the reason why @marc is unable to select items, but I think it is highly likely to be. If I had a dollar for every time I told someone, "Never freeze or turn off Layer 0," I could retire.