I work on Win7 32 bit system.

Please note that the developers specifically say that win7 is no longer supported, so even if there were a problem with Notepad++ specifically under Win7, the developers will not address it. The frequent contributors to the Community Forum take their cue from that, and if it really is a Win7 only problem, we will suggest trying on a supported operating system.

if this is an issue of Notepad++ or of the plug-in itself

Since Notepad++ itself doesn’t have a multi-line toolbar feature, I would hazard a guess that the problem lies in the plugin’s implementation, not in Notepad++ itself. But the plugin author or maintainer would be more likely to be able to investigate and addrses such problems. The author might still be watching the project page for new bug reports or discussion, though it’s been a year since those were active, so…

would be better to have a Customize Toolbar feature directly into N++

The Notepad++ developer has decided that such functionality is better implemented in a plugin rather than as part of the core application, and does not often “compete” with actively-maintained plugins.