They got 140,000 votes in March leaving any vote for socialists parties in the dust.
In the May Euro election, indeed. But there's a lot of caveats that go with that. Foremost, you're comparing a Euro election with Holyrood or Westminster elections. They do a lot better in Euro elections. UK-wide, in the last Euro elections (2009), they polled 16.5%, in the following year's general election they dropped back to 3.1%. In Scotland in 2009 they polled 5.2%, in 2010 Westminster GE they polled 0.7%.
Comparing the 2014 Euro elections, they polled 27.4% UK-wide, and 10.5% in Scotland.
From 2009 - 2010, UK-wide that's a percentage decrease of -81.2%. In Scotland from 2009 - 2010, they showed a percentage decrease of -86.5%. Were they to repeat that in Scotland in the 2016 general election (assuming all other things are equal, which they aren't - the referendum is not a normal event, whatever the outcome) we'd expect them to poll about 1.4% in Westminster elections in Scotland.
Next, remember that the SSP and other left parties didn't stand in the 2014 Euro elections in Scotland. Indeed the only left party that did was No2EU. No2EU only stands in Euro elections, and it isn't well-known here, nor was its purpose well understood. (Anecdotally, I spoke to several people who assumed it was a right wing party).
There are other caveats, but I'm bored now, and you probably are too!