Knowing the make of the reg would make this a lot easier.
Could be leaking from between the bottle and the reg. There is an o-ring on the top of the reg bottle thread. That may need replacing or reseating.
Burst disks. These would be unlikely since they would vent the entire bottle pressure rapidly not slowly leak. It is possible thought that the air is leaking past the disk if the retaining nut is not torqued down fully.
Fill nipple. fairly easy to test, just put your finger over the end and see if the pressure builds up. Remove it, rebuild / replace it.
Leak thought the reg face. This would signify an internal o-ring issue. refer to the manufacturer manual an rebuild the reg.
A few common sense things:
Make up a water and washing up liquid mix. squirt it onto the reg and you will see bubbles from where the leak is.
Lastly and by far the most important, do not out 3in1 oils, etc into your reg. Always use manufacturer specified lubes !!
Ed