Hmmm, rennet comes in different strengths. If you're using liquid rennet and you are following a recipe, or make protocol, and you are using a rennet of different strength, that could be the problem. I have 3 different rennets, one is 750 IMCU, one is 280 IMCU, and the last is 65 IMCU. I use 0.6 ml of the first, 1.6 ml of the second, and around 7.5 ml of the third to get a floc time in the 10-15 minute range. (IMCU is International Milk Coagulating Units). So, if your rennet is around 100 or so IMCU, and you are following a make where the person is using a rennet around 280, and you haven't increased the amount of rennet you're using, that would be the problem.
Check your rennet bottle and see if it indicates the strength in IMCU, or call your supplier and see if they have the information if it is not on the bottle. An internet search may find it too.
If you can find it, check the Library board here under software, and get the CheeseTools.xls workbook (it's for excel). It has a page for calculating how much rennett of a given strength you will need. It should get you in the ballpark, but you may need to adjust up or down (do this by keeping good notes and if your floc is more than 15 minutes, use more, and if less than 10 minutes, use less the next time you make that cheese - there's a page for floc calculations too).
If you're using tablets, I can't help much there as I've never used them. I think if you mix them in water well in advance of when you need to use them the rennet strength decreases, so you should just add the water just before you need to use it, stirr, and then dump it in. Don't be too prepared!
Anyway, if it took 40 minutes to floc, double your rennet for the next make. Should be close to right next time.