Nanostation M5 loco blocked by Window?

I'm trying to setup a point to multi point network and I'm having trouble getting the signal to establish when the units are mounted inside of the building windows. We tested them in an open field and they worked fine. Is it possible that the windows have something in them that is blocking the signal? They are double pane windows. I know 5GHz doesn't penetrate as well, so I'm thinking that may be the case. We are trying to mount them inside because running new shielded/grounded cable isn't really an option. The buildings are only about 1/4 and 1/2 miles apart, so even with a greatly reduced range we should at least get a connection. At this point we are getting either a VERY weak connection or no connection at all.

If it does turn out that we need to mount them outside, can we get away without shielded/grounded wire? I ask this because the cost to re-run the wiring is way more then the cost of this entire project. The POE adapter does have a ground so we don't need to worry about ESD from the internal power or networking. The amount of exposed wire if we did go outside would only be a few inches.