To some extent they're victims of their own success.
First, let's note that the NRA's positions are out of sync with the positions of NRA members, such as on background checks--NRA members support expanding them, the NRA opposes expansion. So they may be bleeding members over that.
But more importantly, mass shootings are the NRA's bread and butter--they're a lobby group for gun manufacturers, not gun owners, remember. Every time there's a mass shooting, the NRA puts out fearmongering about how the government's going to take your guns, and people get scared and buy more guns, which makes the gun makers richer. But there've been so many mass shootings, and so much inaction (except sometimes in states run by Republicans, where they end up loosening gun restrictions, I guess because they were stopping all those good guys from getting guns), that now people see a mass shooting and don't think that the government is going to take their guns, since the fact pattern is established that the government won't do that. Hence mass shootings no longer make money for the gun manufacturers the way they used to, and hence the NRA doesn't get as much money from said companies.