...Does this have to do with operating inside?
Yes. Or at least, very probably.
The tuning control only adjusts the resonant frequency. The impedance
at resonance (and hence the SWR) depends on the design of the loop.
If it is coupled to any metal (which is very likely due to the wiring, ducting,
etc. in a modern apartment) the impedance will change, and you won't be
able to get a low SWR by just adjusting the tuning capacitor. You may be
able to modify the loop feed coupling to adjust accordingly, but the value
will change when the loop is moved or rotated.