Most likely the seals, felt washers, etc inside the Rams have become hardened. This is due to age, wear, weather (especially the sun) salt water, etc. usually just putting a couple of squirts of transmission fluid and operating the unit up and down a number of times will cure this problem. The hydraulic pump system uses almost 5 times more pressure when lifting the lower unit. The lower units weight alone helps to lower it so less ft. Lbs. are needed. Because of this when the washers, seals, etc. get hardened they tend to make it difficult or impossible to lower the unit. The transmission fluid will help lubricate these seals, etc. But this may be temporary. If so then you will need to rebuild the Rams using rebuild kits available from Mercruiser. This is not a job for everyone. So you may have to take the boat to your dealer if a rebuild is necessary. I had this same problem on my Bravo 3. It's been a year since I applied the transmission fluid and cleaned up black plastic, etc. from each Rod. So far it's good. I hope this helps.
