The most likely cause for rez timeouts is that the prims are being created in a place that doesn't allow them. For example, if you are trying to rez the build on a small piece of land, then part of the build might exceed the dimensions of the land. Maxport could be trying to rez one or more prims on one of the neighboring pieces of land. If those neighbors have no build turned on then this can cause a rez failure.
You can minimize this chance as much as possible by placing your avatar in the center of your land.
Also, the new
--lsl
option might be helpful. When you specify the lsl option, Maxport doesn't rez the build directly. Instead, it creates a single prim and uploads the prim data into notecards that it puts in this prim. Then, when you login with the viewer, you can touch the prim to have the build rezzed by the prim. The reason that this might be helpful is that the prim rezzer creates the prims at the position of the rezzer and then the prims move into their final position. Since the prims are created where the rezzer is, they will always be on your land when they rez, so this should eliminate rez failures due to no build.
See this post for more information on the lsl option.
A less likely cause for a rez failure is that the land doesn't have any more prims left in its allocation. This is less likely than the no build scenario because Maxport checks this during its safety checks. But if something else is creating prims at the same time that Maxport is running, it is possible for the parcel to fill up before Maxport is finished.