Put simply, powerline networking is a bit crummy.
In ideal circumstances it works OK, but if (for example) the two units are on different ring mains then performance drops off, and if on the same ring main but subject to electrical noise then performance again drops off. If it's a reliable 75 Mbps, and you don't want to spend any more money you may want to put up with it. Sometimes changing the socket that the distant adaptor is plugged into can help if you can find one on the same ring main as the primary unit, but if it's a noisy circuit that won't help.
Personally I'd never recommend powerline networking because it is so variable. If you want better speeds then there's options like a mesh wifi system, or an Ethernet cable connected access point. A single access point is cheaper than a mesh system and a little bit easier to setup, but needs that ethernet cable between hub and the access point, and often running a cable neatly and securely between the hub and access point is far, far easier said than done.
