not familiar with altium - taking a stab in the dark
since you're looking at a pcb:
if you have the schematic, check that each schematic component is represented by one component on the pcb
check that each component is actually one component and not a "special" component
actually consisting of two parts and created for some obscure undocumented reason
if this is the case, you may have to re-create both components
does it have components on both sides, and you're selecting a component on the top
and a component on the bottom at the same time?
if this is the case, adjust settings so you only select components on the top or the bottom
and not both.