I am using FeynCalc 9.01 in Mathematica 10.4 (The same occurred in FeynCalc 9.0 in Mathematica 10.3).
OneLoop behaves apparently inconsistently as follows :
example =
FeynAmpDenominator[PropagatorDenominator[Momentum[q1], 0],
PropagatorDenominator[Momentum[-k1 - k2 - k3 - k4 + q1], 0]]*
Pair[Momentum[k1], Momentum[q1]]*
Pair[Momentum[k2], Momentum[k1 + k2 + k3 + k4 - q1]]*
Pair[Momentum[p1], Momentum[-k1 - k2 - k3 - k4 + q1]]*
Pair[Momentum[p2], Momentum[q1]];
OneLoop[q1, %, OneLoopSimplify -> True]
produces an error
FCApart::checkfail:
Error! Partial fractioning of the loop integral FeynAmpDenominator[PropagatorDenominator[Momentum[q1, D], 0], PropagatorDenominator[-Momentum[k1, D] - Momentum[k2, D] - Momentum[k3, D] - Momentum[k4, D] + Momentum[q1, D], 0]]*(-Pair[Momentum[k1, D], Momentum[q1, D]] - Pair[Momentum[k2, D], Momentum[q1, D]] - Pair[Momentum[k3, D], Momentum[q1, D]] - Pair[Momentum[k4, D], Momentum[q1, D]])^2*Pair[Momentum[q1, D], Momentum[q1, D]] by FCApart has produced an inconsistent result. Evaluation aborted
1/2 (-1-Pair[Momentum[k1,D],Momentum[q1,D]]-Pair[Momentum[k2,D],Momentum[q1,D]]-Pair[Momentum[k3,D],Momentum[q1,D]]-Pair[Momentum[k4,D],Momentum[q1,D]])
However
example /.
{ -k1 -k2 -k3-k4 -> -b,
k1 +k2 +k3 +k4 -> b};
OneLoop[q1, %, OneLoopSimplify -> True]
produce a result.
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