My opinion is that you have an estimation problem. It seems that the time to test each feature is missing, and only the building part is being considered when planning the sprint.
I’m not saying it is an easy problem to solve, because it is more common than anything. But things that could help are:
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Consider QA as members of the dev team, and include them in the sprint planning and estimating more closely.
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‘Releasable Dev tasks’ should not take up most of the sprint. Complete working features should. Try to gather metrics about dev time vs QA time for each kind of task and use those metrics when estimating future sprints.
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You might need to review your backlog to see if you have very coarse grained features. Try to divide them in smaller tasks that could be easily estimated and tested.
In summary, it seems that your team hasn’t found what its real velocity is because there are tasks that are not being considered when doing the estimation and planning for the sprint.
But in the end, estimation inaccuracy is a tough project management issue that you find in agile-based or waterfall-based projects. Good luck.