I joined the local Padron

Found this article written by a lawyer might be of help to you it states you do need to renew near the bottom of article.

The padrón

The padrón is the certificate that enables you to apply for a SIP (health card) and other local benefits within your town such as enrolling your child in a local school, obtaining a bus pass and using the local library.

To obtain your three-monthly certificate, first you must register. You can do this at a town hall office. You will need to take with you:

• Proof of where you live e.g. rental contract or Title Deed
• Passport
• Residencia – in the case that you apply for this first in your local area
• The last water bill and electricity bill (now asked for by some town halls) 

Once you are registered then the certificate you are issued with lasts for three months. After this you can return to the same office and they will print off a new one for you if you take your passport and previous padrón.

You should renew your registration on the padrón every five years in order to ensure that your name is kept on the register. Town halls may contact people who have not had their certificate reissued for years and ask them to reaffirm their presence in the area. This is a quick process and helps them to confirm who exactly is living in the town.