You make no mention of what sort of culture you are using. When I first started out, I used storebought buttermilk as my acidifying culture, and I believe that it was simply too weak, and did not sufficiently colonize the milk in a reasonable time, leading to a fragile curd that broke apart when I attempted to stir it.
I do not know if this is your problem, but my experience sounds very close to yours.
Butterfat content | U.S. terminology |
80% | Butter (http://cheeseforum.org/wiki/Butter) |
40% | Manufacturers cream |
36% | Heavy whipping cream |
30 – 36% | Whipping cream or Light whipping cream |
25% | Medium cream |
18 – 30% | Light, coffee, or table cream |
10.5 – 18% | Half and half (http://cheeseforum.org/wiki/Half_and_half) |
3.25% | Whole milk |
about 2% | 2% or Reduced fat [6] |
1.5 – 1.8% | Semi-skimmed |
about 1% | 1% or Low fat [6] |
0.5 – 0.0% | Nonfat or Skim or Skimmed [6] |