1. The Transaction has not confirmed yet. Check the receive address you used (or better, the transactionID you should see on the exchange for your withdrawal) on a block explorer like "blockchain.com" or "btc.com". Double check that it is actually confirmed.
2. The exchange has not sent your coins yet. Make sure that the transactionID from the exchange shows up on the blockexplorer.
3. Your Coinomi wallet is not in "sync". It could be they are experiencing network issues and your wallet just hasn't synced properly.
4. You have used an address for a different "coin". Ie. you used a Bitcoin Gold, or Bitcoin Cash address. Coinomi is a multicoin wallet, and I'm sure the exchange you used supports multiple coins. Are you 100% sure that you used a BTC address and purchased BTC?
5. The address you pasted when doing the buy on the exchange was not yours. You might have malware that modifies address in your clipboard when you do copy/paste. Try and copy/paste this address: 1BvBMSEYstWetqTFn5Au4m4GFg7xJaNVN2 and make sure that it pastes the same address. If the address is different, your computer has been infected and you'll need to correct that.