Just let it sit there for 5 or 10 minutes and watch the reading. It should stabilize at some point. If it is drifting wildly, you may have bad tubes. I've seen that with some EH fake Mullard brand EL34s that would drift all over the place and even go into redplating.
As I mentioned, wall voltage can change drastically throughout the day. If you are having to bias every day, you need to measure your wall voltage and make sure you note the bias at that particular voltage. I suspect it is fluctuating.
Also, tubes can change quite a bit during the initial break-in period, which is why many amp manufacturers do a 24-hour burn-in to let everything settle in. If your tubes are new, bias them up, play for a few days, and then re-check, again, making sure they wall voltage is the same as the first time (amp manufacturers use a variac to set the exact voltage every time).
However, you are probably worrying too much about it too much. It just isn't that critical to constantly be trying to hit one magic bias number and keep it there. If the bias current jumps way up and the tubes begin to redplate, you likely have a leaky coupling capacitor or a bad tube, but I doubt that in this case.