So here's the deal. I did the house mission and blew the BOS bunker up. Got back to 38 and grabbed Veronica she had no problems joining me. Did some other quests quick then went back to do the Substation override mission. I get to the casino floor and Veronica's gone however Cass says that I still have a follower and can't take her for a 3some. I've checked the suite, casino, lounge and strip for her and can't find her. Checked the 188 and even went back to the BOS and can't find her anywhere, I can get that she'd be mad at me even though she left the BOS but little bothered by the fact I can't pickup another follower now. Anyone seen this or have any suggestions? (closed and restarted game and still missing)

Does she not appear on your world map as a little triangle ?
Try searching places she was last with you using the local map on the Pip-boy, so you dont have to wander for ages?
If your on PC I can remember others talking about a command prompt that brings followers to your location, I would give you this butI use Xbox and dont know it exactly.
Hope something here was slightly useful.