No only are you potentially running into some of the issues pointed out above (lack of AS/AA feeder flight to gateways and/or the current limitation of only one partner being allowed on a AS award redemption), you could also just have an issue where you are looking for too many seats on the same flight which also limits your results.

BA and Qatar seem to release more seats to be booked with AS miles compared to some other AS partners where they are capped, but you're going to pay for them [BA in taxes/carrier imposed surcharges and with Qatar paying in miles as they're (semi) dynamic] so that might be why only those options are showing up for you.