I don't know which maps you've been looking at, but for this kind of thing I usually use the ones at the National Library of Scotland site -
http://maps.nls.uk/os/6inch-england-and-wales/index.htmlI've found that the 19th century maps often don't show the names for many of the smaller streets, so you need to find them on a later one, and then compare it with one for the relevant date. The ones from the 1930s are often the most useful - there are lots of street names, and this was before the bomb damage of WW2.
However... I've just tried this with Portsea Island and not spotted Plymouth Street. Maybe if you're a bit more patient you'll have better luck. Another useful strategy is to find the road on a census return and see what other roads are nearby - eventually you're bound to see one of them.
If all else fails, see if you can get hold of an even larger scale map (eg 25" to 1 mile). This is the basis for the Alan Godfrey series of paper maps, and there may also be some online.
Arthur
