If you check this video it would let you know the correct way to position the antennas.
Check if you have it correctly. I used to use a Mavic air 1 and the controller looks very similar (if not the same) as the spark. I must say that I also felt cheered on how far it would go compared with what it was supposed to. There was some other things that I did to improve a little the range, like you mentioned, flying with out any obstacles and trying to find the highest point possible.
Also, the channels that I used was a big deal. Like the spark, the Mavic Air 1 has this channel setup that you can set it to auto or manual. With two frequencies. Please check this video where it explains the differences and when you should be using 2.4GHZ vs 5.8GHZ. Each has its benefits and their cons.
Apart from that, the other only thing I would ask you to check, is how far can you go without the controller. Can you check to connect directly from your mobile without the controler? This might give you longer or shorter range. Worth a try.