The issue is the difference in the fitnesse response xml.
The jenkins-fitnesse-plugin retrieves the fitnesse-test-page from the pageHistoryLink xml-node which is missing for fitnesse-tests

Example fitnesse response for suite:
<testResults>
<FitNesseVersion>v20140418</FitNesseVersion>
<rootPath>TestCases.IsatRestAdaptor</rootPath>
<result>
<counts>
<right>96</right>
<wrong>0</wrong>
<ignores>0</ignores>
<exceptions>0</exceptions>
</counts>
<runTimeInMillis>16830
</runTimeInMillis>
<relativePageName>IsatReadOutAdaptorVersions</relativePageName>
<pageHistoryLink>TestCases.IsatIncluded.IsatReadOutAdaptorVersions?pageHistory&resultDate=20150112124944
</pageHistoryLink>
<content>
...
</content>
</result>
<finalCounts>
<right>1</right>
<wrong>0</wrong>
<ignores>0</ignores>
<exceptions>0</exceptions>
</finalCounts>
<totalRunTimeInMillis>0</totalRunTimeInMillis>
</testResults>

Example fitnesse response for test:
<testResults>
<FitNesseVersion>v20140418</FitNesseVersion>
<rootPath>TestCases.IsatIncluded.IsatReadOutAdaptorVersions</rootPath>
<result>
<counts>
<right>96</right>
<wrong>0</wrong>
<ignores>0</ignores>
<exceptions>0</exceptions>
</counts>
<runTimeInMillis>15797</runTimeInMillis>
<content>
...
</content>
<relativePageName>IsatReadOutAdaptorVersions</relativePageName>
</result>
<finalCounts>
<right>1</right>
<wrong>0</wrong>
<ignores>0</ignores>
<exceptions>0</exceptions>
</finalCounts>
<totalRunTimeInMillis>16355</totalRunTimeInMillis>
</testResults>

Code changed in jenkins
User: Antoine Aumjaud
Path:
.gitignore
src/main/java/hudson/plugins/fitnesse/FitnesseResults.java
src/main/java/hudson/plugins/fitnesse/NativePageCounts.java
src/test/java/hudson/plugins/fitnesse/NativePageCountsParserTest.java
src/test/java/hudson/plugins/fitnesse/NativePageCountsTest.java
http://jenkins-ci.org/commit/fitnesse-plugin/cf0644c8fc4dbd377f398d01e2d1db785c052ad6
Log:
Merge pull request #22 from antoine-aumjaud/features/FITNESSE-PLUGIN-FixGetDetailOnTestPage

Fix JENKINS-26387: can't run a test page only

Compare: https://github.com/jenkinsci/fitnesse-plugin/compare/19b742134f0d...cf0644c8fc4d