Your post is lacking enough info to make any sort of diagnosis. There are so many factors that come into play here, it is almost impossible to say where the problem lies. Get a good oil heating man in there to check it out.
All of these types of tankless coil setups are plumbed with the tempering or mixing valve most likely like this. http://seek.autodesk.com/product/latest/agg/watts/1/Watts/717
This type of valve usually fails within 5 years. There is a repair kit for it, but its frequently easier to replace with a better valve. Have a professional oil guy or plumber repair or replace the valve. Also my advise is to look into upgrading to an indirect tank, as coil is the most expensive and least efficient way to get hot water these days.
Very true too much info is missing. One would have to figure out if the coil is plugged up with deposits, etc and is no longer effectively heating up water. However I have seen quite a few cases where the cheap Watts 70A's fail after just a few years. My own house had a coil when I first moved in, the mixing valve was so corroded in place you coulnd' turn the knob on top. Replacing the valve fixed the problem for the short term, still the hot water supply was not great at all. Ulitimately I went to an indirect tank anyways...