IOTA’s tangle seems facing synching issue right now as it uses unproven technology called DAG(directed acyclic graph) and coordinator for its consensus mechanism.
Compared to blockchain’s linear chaining topology tangle’s exponential tangle topology supposed to outperform the scalability of Bitcoin blockchain, more secure and flawless however it begins to create greater issue than scalability.
Source:https://blog.iota.org/on-the-tangle-white-papers-proofs-airplanes-and-local-modifiers-44683aff8fea
However the possibility of having separate tangle (like the fork of bitcoin blockchain) increases as the exponentiality of tangle multiplies as the tangle of IOTA grows. In order to solve this problem IOTA foundation designed ‘tip sel’ (Tip Selection mechanism) which enforces the selection of a different chunk of tangle to be integrated into main tangle however this seems not working as of July 7 of 2018. Synching between main tangle and side tangle seems not working at the moment.
Source:https://www.reddit.com/r/Iota/comments/8wrql2/announcement_regarding_side_chain_and_syncing/
To make this issue even more complicated it has never been like this before according to IOTA development team of the foundation. The key issue seems that you can not force tipsel of specific separate tangle. IOTA’s full node called coordinator supposed to select which tangle to be the maintangle however IOTA’s algorithm on this selection seems not as transparent as that of 51% rule of bitcoin blockchain.
To improve the transaction speed and scalability issue IOTA employed new technology however it was not fully tested and proved until they released it. The burden of using proven technology can kill the entire IOTA’s ecosystem even before it blooms.
With carefully coordinated use of open source and social coding it might have prevented however IOTA has been too secretive just like Peenemunde top secret research center. That is betraying the very idea of bitcoin which makes bitcoin decentralized and autonomous out of few evil hand which manipulates and cheats everybody.
Source: https://en.wikipedia.org/wiki/Peenem%C3%BCnde_Army_Research_Center
As you may aware IOTA foundation never released the source code of their coordinator function and the current synching issue seems to be directly related to the algorithm they applied to tipsel for selecting separate tangle. 1)
The possible cause of the issue can be traced back to the recent upgrade of new tip selection algorithm they have applied very recently here. 2) Go find it yourself.