We have a late 20's brick bungalow with plaster walls. (I'm not entirely sure they are lathed - some of our internal walls had plaster attached to rock lath/gypsum board, but I'm not sure this was used on exterior walls?)

We have a large bow above one of our windows, visible to the eye and only the plaster (the exterior wall structure is sound). The window is still straight and plumb, however the window trim on the wall has pulled away.

It began to happen some time ago, as earlier owners filled the space with caulk or putty of some type . I'm not entirely sure it's getting worse (we've only had the house for 6mo) It's not mushy or soft - the plaster itself seems to be in fine shape. There are no visible cracks in the plaster, only a slight hairline crack in the paint that we applied when we moved in.

Any ideas on what has happened? I suspect the lathe (or whatever was used as a base for the plaster) has pulled away from the brick). Is this something we need to fix posthaste? Should we just keep an eye on it and see if it gets worse?

2-coat plaster (16x48 inch gypsum base) was used on all walls.

In masonry houses interior plaster was often applied directly to the masonry of the outside walls (special blocks were made with grooves molded in to allow the plaster to bond).

A large area of plaster that has come loose can be fastened back down if there is wood in the wall (studs) to drive long enough screws into.

Plaster washers and long enough screws are used to pull the plaster back tight, and then the area is covered with another coat of plaster to conceal the washers.

If the bulge is more than about 1/2 inch high it is likely to need to be removed, the substrate repaired, and then the area patched.

Setting type joint compound (Durabond or Easysand) actually work well for repairing plaster.

A bonding agent is still required (just as with plaster) to prevent moisture form being pulled from the patch before it sets correctly, but setting compound mixed about as thick as peanut butter (smooth please) has very little shrinkage as it sets (even in thick and large repairs).

White Elmer's glue cut 50% with water and then painted on makes an excellent bonding agent.
Just paint it on old plaster edges and surfaces, allow it to dry to tacky, then put setting compound over the area.

The first coat does NOT need to be smooth, just below the final desired surface by about 1/8 to 1/4 inch.

Further coats will smooth out the repair and make it level and even with the old surface at the edges.