In some cases, when a NOT node is the root of a subnetwork, the phreak segment having that node as root is not initialized as expected when the initial fact is inserted. That can prevent a rule to fire even if it should as demonstrated by the following test case.

    @Test
    public void testNestedNots() {
        String str = "package org.test; " +

                     "rule negation_distributed_partially_no_sharing " +
                     "when " +
                     " (not String()) " +
                     " and " +
                     " (not (Double() and Integer())) " +
                     "then " +
                     " System.out.println(\"firing\"); " +
                     "end";

        KnowledgeBase kbase = loadKnowledgeBaseFromString(str);
        StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();

        int n = ksession.fireAllRules();

        assertEquals(1, n);
    }

Note that if you invert the 2 NOTs putting the one with 2 conditions before the other the test succeeds.