TA problem here too, with United
Having a problem myself with tickets bought from a TA (Travel Up (UK).
Booked 2 returns with United Airlines, Glasgow (UK) to Newark (NJ), in March 2018, and a day later United withdrew all winter flights from Glasgow, apparently offering a refund or alternative scheduling.
I asked my TA for a refund and despite paying £1075 for the flights, the TA says that United will only refund me £921. The TA is less than helpful when I ask them to explain why I am not getting a full refund and currently I am stuck! Looks like I'll need to get a lawyer involved.