Buying competent SEO services for your chosen good keywords is a great idea.
Do research on which keywords you need to promote and then ask your seo service to work on those lines. Choose a very competent seo service. If it is providing you the keywords, then do a reseach on them (keywords) first. Not all keywords are made equal, and they can even harm your site in extreme cases, or make you lose money needlessly.
May be you'd appoint a seo company that's either not too competent or is too busy to carefully choose the keywords (and choose a top-quality line of action), or have some newbie on the payrolls, still not an expert, who'd be (unfortunately) given the task of beginning your campaign... whatever... you must have some control right from the beginning of a seo campaign.
Take help or read more, whatever that needs for you to understand what's going on with your campaign... money alone doesn't bring success, it's the wise/expert mind that controls its outflow and that supervises the campaign flow will help bring a positive RoI. IMO.
May be your competing pages are well set for those keywords, they've been taken care of by better SEO and money power, and the competitors are quite active and won't allow you to have a higher place... Find other powerful keywords in that case and see if those too are having that kind of a competition. Anyway, I think that 4 months can be a very short time to look for any success for certain keywords.
P.S.:
May be you need to wait some more time to see if the effect works. Talk with your SEO service and ask why isn't there a positive result. May be you'll understand how their system works for their clients, and whn you could see better results. May be they can fine tune the campaign so that any existing blocks can be removed... They could be busy on other things, so speak with them.
