It appears that the mechanics in this game are completely unmerciful to the losers. Once you start to lose your footing (research points, high ground positions, etc.) it seems to lead to a slow and painful death. I may have only played a few games but I haven't seen any big comebacks from a losing position. So I ask, what are some things you can do to stop your team from losing and start gaining some ground?
Once you are contained in the base, you are basically dead. Spam surrender until the team accepts.
However, before that, it is possible to return from a disadvantageous position - but of course that is not easy and requires a good team.
Build around the enemy away from the fighting and try to get a spawn up behind the opponent. Reinforce it with a powerplant so it can't be cut easily. Have as many of your players as possible spawn there with siege exos and focus on destroying relays and powerplants to cut power so the opponent can't advance and has to spawn from further away.
In the meantime your stealth players have to make sure you always own the secondaries (assuming you can't take primary). Having both secondaries puts you on almost equal footing in the resource income.
Depending on the map, you can try to get a secret spawn up close to their base (good on hydro, oasis and silo), starting with your flanking spawn and start sieging their base. It can throw the opponent into confusion and often ends up in the opponent overcommiting to defending or losing critical buildings (first target: assembler), allowing you to capture primary and secondary nodes.
However, yes, the game is unmerciful and if you are behind, it is really, really hard to come back against a good team. It is possible, i've done it several times, but you need good coordination (use voice chat) and your team needs to obey the orders, not run around looking for kills. Try to always have either have primary or both secondaries and send stealth (in mid- to lategame) out to cap if you don't, otherwise everything will go downhill fast.