What do you mean by "waterproofed"? Where? Above and/or below grade? And with what?

We dug along the wall (deep, to the foundation), then used this sealant and plastic sheeting. to waterproof the wall. We have used this technique to fix a similar problem in another location previously with good result.

A dimpled membrane designed specifically for the purpose, from DMX or Delta, for example, would be a surer bet. If possible, replace footer drains and back fill the first few feet with sand or gravel.

I doubt the house has footer drains. Is it expensive to put them in? If yes, is there an alternate (and inexpensive) solution?

Footer drains run around the entire perimeter of the house and drain water either to daylight 20-30 feet away or to an interior sump that pumps to exterior drainage, or, if allowed to a municipal storm system. Messy and costly in an existing house.

Building Science Corp.

An alternative is interior drains, which, in effect do the same thing, but on the inside of the basement. Less expensive, but interior finishes have to be removed and replaced.

It may be possible that just excavating the new area of leakage will cure the problem. We did that for about 30 feet on one home and that worked--likely because the excavation revealed a small foundation crack that was the major source of the water.

Always run a dehumidifier when the relative humidity is above 40% or so.