With any Presta tube I've always twisted them open a full turn, depressed the stem with my finger to break the seal, jammed the Silca head on the stem and pumped away.

If you hear air rushing out of the Silca Head, it might need to be screwed down a little tighter ... that expands the rubber washer for a better fit.

If you hear air rushing out of the base of the pump, the hose and seal might need to be tightened a little.

If you hear air escaping towards the top of the pump, the leather washer is drying out. Just a little dab of oil on the leather works for me.

Other than that, the check valve in the base might be sticky. Usually it only gets that way if you overdose the leather washer with oil and the sludge accumulates around the needle valve ... easy to dissasemble and clean in any event.