A study on why Venus is elevated and powerful in Pisces and weak in Virgo:

The Kama triangle is 3rd, 7th and 11th houses. The Kama may be described as either or a combination of love, sex and pleasure. The way of living through pleasure is ‘The Kama’. The Kama, the primary feebleness of the human mind, is both an effort and one of the six enemies of the human mind (Arishadvarga or Shadripu) is also an important factor of conjugal life. The 6th house is called Ripu Bhav or Roga or the Disease, and the Kama is the prime of diseases. 11th house is the most strong in the Kama triangle and holds the key to a happy conjugal life. As Venus signifies the seed, it is the factor of the Kama. Pisces is 6th sign from Libra and 11th sign from Taurus, hence Venus is the most powerful in Pisces because Libra and Taurus are respectively the original triangle and own sign of Venus. The peak of the power position of Venus is 27 degrees in Pisces in Revati Nakshatra. The presiding deity of Revati Nakshatra is Pushan, one of the 12 Adityas and the deity for the marriage ceremony, and he represents semen. Although Revati Nakshatra is considered as Moksha giving, it is also the most favourable Nakshatra for a happy married life. That is why Venus gets the most power in this Nakshatra.

Aries, Gemini, Leo and Virgo are called barren signs, and Gemini and Virgo are the two signs of Mercury, especially Virgo is the most barren as it is the active sign of neuter lord Mercury. Mercury is considered a child planet in Astrology, and when the Kama causing Venus comes in the active sign of Mercury, it loses the importance and power. The weakest position is 27 degrees in Virgo, which falls in Chitra Nakshatra. The ruling deity of Chitra Nakshatra is Tvastar, the artisan god. Since he created many weapons to defeat the demons, Shukracharya (Venus) does not tolerate him thus, Venus loses strength in Virgo in Chitra Nakshatra.