Rennets vary in strength, and are sensitive to storage. Assuming that yours has been stored properly, are you acidifying the milk properly before adding rennet? Most of the time your cultures need time to work before the milk is acidified enough for rennet to work.
Why not test using 1 or 2 liter batches, heat, add culture, let it work, then rennet at 2-4 times the recommended amount. If that works in a reasonable time, then you can adjust the amounts you use. If not, I'd ask for a replacement. The rennet I have says use 1 tsp to set 2 gallons, but I find I need to use only 3/4 tsp for 3.5-4 gallons of milk to get flocculation in 15 minutes.
As you've discovered, "expired" rennet doesn't immediately stop working, you may just have to use a bit more to get the flocculation time you are used to. I can't comment on whether that affects the taste of the cheese.