What exactly does I-P return? Does it declare the problem infeasible? Does it time out? It might help others to post some output snippets.
Is the solution of the problem with only bounds on the variables near the bounds? If so, that might help explain why interior-point takes small steps towards the end.
From the name, you can tell that the interior-point algorithm wants to remain on the interior of the bounded region. If the solution is near a boundary, it will approach slowly and may never fully make it there. By construction, the inner workings of the I-P algorithm are unstable near constraint boundaries.
The above may also explain the lack of convergence for the inequality constrained formulation.