Shooting indoors you're encountering much lower contrast light, and you're apparently used to contrasty light outdoors. It is also possible you're underexposing the film indoors.(posted 9077 days ago)If you shoot in flat light, you'll get flat negs. You could try increasing the development of the neg to increase contrast, but this assumes you're shooting an entire roll under the same lighting conditions. You can also try increasing the contrast while printing.
Double check to make sure you're not underexposing, perhaps try bracketing down a couple of stops and see if you like the results better.
Dana