Wood (Timber) was typically used for beams and columns in older homes. These beams may be inadequate to properly support the building loads being imposed on them. Over time the timber beams may deflect or bow downward under the excessive loading. Other signs of an over loaded timber beam include crushing of the wood at column and wall locations, and the development of significant cracks. Significant cracks would be cracks all the way through the timber beam which is different than normal surface checking.
In some cases over stressed timber beams can be reinforced with new support columns installed between the existing columns. This reduces the span of the beam which reduces the stress levels. If the beam is deflected or bowed down, you may be able to recover 50% to %75 of the downward movement. 100% recovery is unlikely as timber beams which have been in a deflected or bowed position for a long period of time tend to maintain that deflected shape. If your objective is to get the floors back to as level a position as possible, or if additional support columns can not be added, then replacement of the timber beam is your best option. Replacement is typically done with steel beams and columns.
Another possible cause of downward movement of a timber beam is deterioration along the bottom of supporting timber columns. Often these timber columns are set on top of footings below the concrete floor slab where moisture causes the columns to deteriorate and compress. As they compress they move down and the beam moves down with them.