There is a bug with Series EV where if you upgrade her towers while she is not currently one of the heroes being played, they will not gain the additional HP until she is brought back out. I have attached screen shots of this in the tavern, but the same situation happens in a map as well:
>Edit: just discovered this only holds true if you start from a 0 upgrade state. If you have already upgraded it once before you switch her out it will work correctly
*This shows an un-upgraded wall with series EV still out* http://steamcommunity.com/profiles/76561197992806011/screenshot/56093772625