Some FF programs limit the number of segments that can be included on a single award ticket. Going to and from RNO automatically gives you one segment in each direction, which then for some itineraries would mean that the number of further connections you can take to find better availability is limited. For example, if you're trying to go to PRG, the fewest connections might be a routing something lime RNO-LAX-LHR-PRG (three segments) but you might want to do RNO-LAX-ORD-JFK-LHR-PRG (five segments) to get a TATL award seat on your date, but some programs would not allow this.

Sometimes if this happens or if availability (for saver miles) is limited from your small airport, it can make sense to purchase a separate ticket for that part of your trip, in this case RNO-LAX if there's a cheao RT, perhaps even on a different carrier, but in this case you need to allow lots and lots of time to make the connection and allow for problems.