Write to Lufthansa and state that they are in breach of Regulation EU 261/04 and you demand your compensation be paid within 30 days. On the letter state that you are copying your correspondence to the following agency:
Luftfahrt-Bundesamt (LBA)
DE - 38144 Braunschweig
Tel.: +49 531 – 23 55 115 (mo-th 9-16, fr 9-15)
Fax: +49 531 - 2355707
fluggastrechte@lba.de
http://www.lba.de
This is the competant authority for regulation in Germany. Lufthansa cannot withhold your refund for this length of time, but let them know you mean business. You are fully entitled to your refund. Incidently, make it clear that you dont' just want the refund of the fare difference, but also the compensation due to the cancellation under 261/04, as weather is not classified as an "extraordinary circumstance" under EU law. They are stupid, you clearly thought you were only entitled to the fare difference, this may end up costing them much more.