I can confirm the same behaviour:
1) Take photo when charge at a decent levlel (>20%)
Result: Photo should be streamed.
2) Take photo when charge <20%
Result: Photo will not be streamed
3) Charge iPhone until charge >20%
Result: Photo not yet streamed
4) Charge iPhone until charge >20% AND THEN access the photo app and click the Photostream album
Result: Photostream kicks in and starts streaming the photo(s) taken when the charge was <20%
Not sure why step 3) didn't automatically stream - maybe a coincidence that it started to stream as I accessed the app.
Photostream only works if both the the sending and receiving device are connected to wifi.
If you take any photos outside of wifi range then assuming battery charge is sufficient, the streaming of photos that were taken outside of wifi range will begin once both devices become connected to wifi.
There does appear to be a random bug that causes the odd photo not to stream - even if all the conditions above are met. There's many reports of this and I've experienced it with just 1 photo out of about 500 (so far!).