DIY Conservatory links

A few months back I contracted a seo company to try and improve Project Conservatories organic web page ranking. After about two months the site had improved from position 250 to about 100 for the key words DIY conservatory, DIY conservatories and self build conservatory. This was achieved by them adding links to other sites. I then started adding more content to the web site and by the end of month 3 the site was ranking on page 3,4 and five. Fantastic. Then boom, the site is sitting near page 30 and nobody has the answer why. The saga continues