Sometimes (just sometimes), decreases are good. Also, sometimes (just sometimes), decreases are not bad. For example, if your website is currently linked on a bunch of bad backlink websites and directories (an SEO scam from yesteryear that some SEO companies still use), it may show increases, but these increases almost always make for poor leads, and random traffic. So, if you hire a professional, that is focused on improving the quality of your website traffic, which is far more important than quantity, you may in fact see a decrease in traffic, but an increase in sales. Also, long term growth is what you're looking for, not just short term bursts in traffic. As long as the long term results are consistent, it might mean you found "one of the good ones."
SEO is long term, so sometimes you will find around a 5% variance in traffic up or down, it's never a reason to get concerned unless it continues month after month. Think of it this way, if your website analytics show 500 new visitors, and you see an increase in sales, but the following month, there is a decrease by 25 visitors; you are still seeing an increase by 475 visitors over the previous months before the increase. The point is, variance is normal, and expected. If that carries on for a few month, now it's something to be concerned about. Sometimes professional SEO's get lazy, and may do things the right way, but eventually focus less on specific "safe" clients, whose rank has been consistent. The problem with this is, it gives a competitor an "in" to bump you out of place. Remember, for every dollar you are spending on SEO or Online Marketing, one of your competitors is also spending that same money (or more).
We never get complacent here at Make it Active, LLC. Instead, we have specific hours dedicated to specific clients every week. No client gets left behind with Make it Active!