Take outside temporarily - pref into area where you can get good line of site to distant points - e.g. hillside and try again there - if still poor then may be a problem but likely its down to materials surrounding you when in attic - have found several times that various types of roofing mineral felt have high metallic content causing screening or if roof space insulated then the insulation material can have foil backing or be a foil sandwich again causing problems, as can plumbing, metal water tanks, roof infrastructure girders, etc. in loft space. I’ve occasionally found mounting high in attic space a problem but then trying down at floor level worked much better as signal was able to escape through the eves of the roof line 
Windows themselves especially more modern double/triple glazed units can often be made with architectural glass or low-K (thermally insulating) glass which is made with thin metal/metal oxide coating that can be quite effective at cutting RF…you may find <Ghz cellular signals also weak up there which gives a clue despite height c/w rest of the building…
If the window opens, again as a temp measure try pushing ant outside with a short RF feeder - 1-3m RG58 ok - you will loose some signal but being high/outside should more than compensate…perhaps holding ant aloft taped to top of 1-2m 15/22mm poly pipe to keep upright in air.