Note: the problem happens with minecart chests / minecart hoppers.

Problem 1 (Loading issue): Basically, if I connect a hopper to many minecart chests in the same spot (loading system), the items in the hopper will only and always flow to the first minecart chest I placed even if the other minecart chests are empty -> it means that if the first minecart chest is full / or fully don't contain the items in the hopper - items will get stuck in the hopper instead of search an available minecart in that spot and flow to it. 

Problem 2 (Unloading issue): If I place a hopper under minecart chests (unloading system), the hopper will only suck the items in the first minecart chest I placed.

It means that items will get stuck in minecart chests instead of unload the items to the hopper below.

Hopper will only suck the items in the first minecart chest I placed - watch video 3 to understand clearly.

The main problem is that hoppers will only search for the first minecart that placed and ignore the other minecarts 

Recommend watching all the videos to understand clearly what is the problem. Ask me anything you want.

In the first video I added a minecart chest in addition to the first 'unavailable' minecart and items didn't flow to the empty one.

In the second video I broke the first minecart and items started to flow to the only minecart chest that left.